CI: switch to GitHub Actions - step 1: code style

This commit:
* Adds a GH Actions workflow for the CI code style check and to validate the `composer.json` file.
* Removes those actions from the `.travis.yml` configuration.
* Adds the new `.github` directory to the `.gitattributes` file to be `export-ignore`d.
* Adds a "Build Status" badge in the Readme to use the results from this particular GH Actions run.

Notes:
1. Builds will run on all pushes and on pull requests.
2. Builds can also be manually triggered.
    Note: manual triggering of builds has to be [explicitly allowed](https://github.blog/changelog/2020-07-06-github-actions-manual-triggers-with-workflow_dispatch/). This is not a feature which is enabled by default.
3. If a previous GH actions run for the same branch hadn't finished yet when the same branch is pushed again, the previous run will be cancelled.
    In Travis, this was an option on the "Settings" page - "Auto cancellation" -, which was turned on for most, if not all, Yoast repos. The `concurrency` configuration in the GHA script emulates the same behaviour.
4. Composer dependency downloads will be cached for faster builds using a [predefined GH action](https://github.com/marketplace/actions/install-composer-dependencies) specifically created for this purpose.
    The alternative would be to handle the caching manually, which would add three extra steps to the script.
    Note: Caching works differently between Travis and GH Actions.
    On GH Actions, once a cache has been created, it can't be updated. It can only be replaced by a new cache with a different key.
    As the PHP version, the `composer.json` and a potential `composer.lock` hash are all part of the key used by the above mentioned action, this difference should not have a significant impact.
    Ref: https://docs.github.com/en/actions/advanced-guides/caching-dependencies-to-speed-up-workflows
5. The CS check will display the results in the actions script output log, as well as display any violations found inline in the GitHub code view using the [cs2pr](https://github.com/staabm/annotate-pull-request-from-checkstyle) tool.

Differences with the Travis implementation:
* This check will now run on ALL pushes and pulls.
    The branch filtering which was previously being applied in Travis has not been implemented for this script.
* The `composer validate` command will now only be run against the PHP version used in the `cs` script (PHP 7.4), not against multiple PHP versions.

.gitattributes

@@ -13,6 +13,7 @@
 phpcs.xml export-ignore
 phpcs.xml.dist export-ignore
 /.cache export-ignore
+/.github export-ignore
 
 #
 # Auto detect text files and perform LF normalization

.github/workflows/cs.yml

@@ -0,0 +1,50 @@
+name: CS
+
+on:
+  # Run on all pushes and on all pull requests.
+  push:
+  pull_request:
+  # Allow manually triggering the workflow.
+  workflow_dispatch:
+
+# Cancels all previous workflow runs for the same branch that have not yet completed.
+concurrency:
+  # The concurrency group contains the workflow name and the branch name.
+  group: ${{ github.workflow }}-${{ github.head_ref }}
+  cancel-in-progress: true
+
+jobs:
+  checkcs:
+    name: 'Check code style'
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v2
+
+      - name: Install PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: '7.4'
+          coverage: none
+          tools: cs2pr
+
+      # Validate the composer.json file.
+      # @link https://getcomposer.org/doc/03-cli.md#validate
+      - name: Validate Composer installation
+        run: composer validate --no-check-all
+
+      # Install dependencies and handle caching in one go.
+      # @link https://github.com/marketplace/actions/install-composer-dependencies
+      - name: Install Composer dependencies
+        uses: ramsey/composer-install@v1
+
+      # Check the codestyle of the files.
+      # The results of the CS check will be shown inline in the PR via the CS2PR tool.
+      # @link https://github.com/staabm/annotate-pull-request-from-checkstyle/
+      - name: Check PHP code style
+        continue-on-error: true
+        run: composer check-cs -- --report-full --report-checkstyle=./phpcs-report.xml
+
+      - name: Show PHPCS results in PR
+        run: cs2pr ./phpcs-report.xml

.travis.yml

@@ -3,7 +3,6 @@ language: php
 
 cache:
   directories:
-    - .cache
     - vendor
     # Cache directory for older Composer versions.
     - $HOME/.composer/cache/files
@@ -18,14 +17,12 @@ branches:
 
 php:
   - 8.0
+  - 7.4
   - 7.0
   - 5.6
 
 jobs:
   fast_finish: true
-  include:
-    - php: 7.4
-      env: PHPCS=1
 
 before_install:
   - phpenv config-rm xdebug.ini || echo 'No xdebug config.'
@@ -41,10 +38,3 @@ install:
 script:
 # PHP Linting
 - composer lint
-
-# Validate the composer.json file.
-# @link https://getcomposer.org/doc/03-cli.md#validate
-- if [[ $TRAVIS_PHP_VERSION == "5.6" || $TRAVIS_PHP_VERSION == "7.4" ]]; then composer validate --no-check-all; fi
-
-# Check the code against YoastCS.
-- if [[ $PHPCS == "1" ]]; then composer check-cs; fi

README.md

@@ -1,3 +1,4 @@
+[![CS](https://github.com/Yoast/i18n-module/actions/workflows/cs.yml/badge.svg)](https://github.com/Yoast/i18n-module/actions/workflows/cs.yml)
 [![Build Status](https://travis-ci.org/Yoast/i18n-module.png?branch=master)](https://travis-ci.org/Yoast/i18n-module)
 [![Code Climate](https://codeclimate.com/github/Yoast/i18n-module/badges/gpa.svg)](https://codeclimate.com/github/Yoast/i18n-module)