chore: Update CI to run Travis + Linux + Chromium

Our CI build has been incredibly flakey across all three of our current
CIs:

* Appveyor: Chromium + Windows
* Travis: Firefox + Linux, Chromium + Linux
* Cirrus: Chromium + Linux, Chromium + Mac

Legitimate issues and errors have been missed because it's expected that
the CI is red and therefore it's not seen as an issue when a PR's build
fails.

We should have a build that covers the full combo of browsers and
operating systems but it's more important to have a consistent, reliable
green build where failures are genuine. So this commit strips our CI
back to Chromium on Linux on Travis, and nothing more. Once this is
stable we will expand out into more operating systems and bring back
Firefox, too.

.travis.yml

@@ -1,4 +1,5 @@
 language: node_js
+os: linux
 dist: trusty
 addons:
   apt:
@@ -10,31 +11,13 @@ notifications:
 cache:
   directories:
     - node_modules
-# allow headful tests
-before_install:
-  - "sysctl kernel.unprivileged_userns_clone=1"
-  - "export DISPLAY=:99.0"
-  - "sh -e /etc/init.d/xvfb start"
-  - 'if [ "$FIREFOX" = "true" ]; then pyenv shell 3.6; pip3 install --user mozdownload; fi'
-  - 'if [ "$FIREFOX" = "true" ]; then rm -rf $FIREFOX_HOME; mozdownload -t daily -d $FIREFOX_HOME/latest.tar.bz --log-level DEBUG; fi'
-  - 'if [ "$FIREFOX" = "true" ]; then cd $FIREFOX_HOME; tar -xvf latest.tar.bz; cd -; ls $FIREFOX_HOME/firefox/firefox; fi'
-script:
-  - 'if [ "$CHROMIUM" = "true" ]; then npm run lint; fi'
-  - 'if [ "$CHROMIUM" = "true" ]; then npm run coverage; fi'
-  - 'if [ "$FIREFOX" = "true" ]; then BINARY=$FIREFOX_HOME/firefox/firefox npm run funit; fi'
-  - 'if [ "$CHROMIUM" = "true" ]; then npm run test-doclint; fi'
-  - 'if [ "$CHROMIUM" = "true" ]; then npm run test-types; fi'
-  - 'if [ "$CHROMIUM" = "true" ]; then npm run bundle; fi'
-  - 'if [ "$CHROMIUM" = "true" ]; then npm run unit-bundle; fi'
 jobs:
   include:
-    - node_js: "12.16.1"
+    - node_js: "10.19.0"
+      dist: trusty
       env:
         - CHROMIUM=true
-    - node_js: "10.18.1"
-      env:
-        - CHROMIUM=true
-    - node_js: "10.18.1"
-      env:
-        - FIREFOX=true
-        - FIREFOX_HOME=$TRAVIS_HOME/firefox-latest
+      before_install:
+        - "export DISPLAY=:99.0"
+        - "sh -e /etc/init.d/xvfb start"
+      script: ./travis/chromium.sh

travis/chromium.sh

@@ -0,0 +1,8 @@
+#!/usr/bin/env sh
+
+npm run lint &&
+npm run coverage &&
+npm run test-doclint &&
+npm run test-types &&
+npm run bundle &&
+npm run unit-bundle