Use GitHub Actions instead of Travis CI (#309)

Travis CI is no longer providing CI minutes for open source projects: https://news.ycombinator.com/item?id=25338983

It's also been pretty slow: the build for #308
hasn't started in 13 minutes

The config file is based on the template at https://docs.github.com/en/free-pro-team@latest/actions/guides/building-and-testing-nodejs#specifying-the-nodejs-version

.github/workflows/build.yml

@@ -0,0 +1,22 @@
+name: Node.js CI
+
+on: [push]
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Use Node.js
+      uses: actions/setup-node@v1
+      with:
+        node-version: '15'
+    - run: npm install
+    - run: npm test
+    - shell: bash
+      env:
+        SAUCE_USERNAME: ${{ secrets.SAUCE_USERNAME }}
+        SAUCE_ACCESS_KEY: ${{ secrets.SAUCE_ACCESS_KEY }}
+      run: '[ -z "$SAUCE_USERNAME" ] || [ -z "$SAUCE_ACCESS_KEY" ] || npm run test-saucelabs'

README.md

@@ -1,6 +1,6 @@
 # XRegExp 4.4.0
 
-[![Build Status](https://travis-ci.org/slevithan/xregexp.svg?branch=master)](https://travis-ci.org/slevithan/xregexp)
+[![Build Status](https://github.com/slevithan/xregexp/workflows/.github/workflows/build.yml/badge.svg)](https://github.com/slevithan/xregexp/actions)
 
 XRegExp provides augmented (and extensible) JavaScript regular expressions. You get modern syntax and flags beyond what browsers support natively. XRegExp is also a regex utility belt with tools to make your grepping and parsing easier, while freeing you from regex cross-browser inconsistencies and other annoyances.