Replace Travis with a GitHub Actions CI workflow

The backend Hypothesis projects have switched to using GitHub Actions
for CI. This does the same for the Hypothesis client.

.github/workflows/continuous-integration.yml

@@ -0,0 +1,23 @@
+name: Continuous integration
+on: [push]
+jobs:
+  ci:
+    runs-on: ubuntu-latest
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v2
+    - name: Cache the node_modules dir
+      uses: actions/cache@v2
+      with:
+        path: node_modules
+        key: ${{ runner.os }}-node_modules-${{ hashFiles('yarn.lock') }}
+    - name: Install
+      run: yarn install --frozen-lockfile
+    - name: Format
+      run: yarn checkformatting
+    - name: Lint
+      run: yarn lint
+    - name: Typecheck
+      run: yarn typecheck
+    - name: Test
+      run: yarn test

README.md

@@ -1,12 +1,12 @@
 Hypothesis client
 =================
 
-[![Build status](https://img.shields.io/travis/hypothesis/client/master.svg)][travis]
+[![Continuous integration]](https://github.com/hypothesis/client/workflows/Continuous%20integration/badge.svg?branch=master)][gha]
 [![npm version](https://img.shields.io/npm/v/hypothesis.svg)][npm]
 [![#hypothes.is IRC channel](https://img.shields.io/badge/IRC-%23hypothes.is-blue.svg)][irc]
 [![BSD licensed](https://img.shields.io/badge/license-BSD-blue.svg)][license]
 
-[travis]: https://travis-ci.org/hypothesis/client
+[gha]: https://github.com/hypothesis/client/actions?query=branch%3Amaster
 [npm]: https://www.npmjs.com/package/hypothesis
 [irc]: https://www.irccloud.com/invite?channel=%23hypothes.is&hostname=irc.freenode.net&port=6667&ssl=1
 [license]: https://github.com/hypothesis/client/blob/master/LICENSE

src/karma.config.js

@@ -13,9 +13,8 @@ let mochaOutputMode = 'minimal';
 
 process.env.CHROME_BIN = require('puppeteer').executablePath();
 
-// On Travis and in Docker, the tests run as root, so the sandbox must be
-// disabled.
-if (process.env.TRAVIS || process.env.RUNNING_IN_DOCKER) {
+// In Docker, the tests run as root, so the sandbox must be disabled.
+if (process.env.RUNNING_IN_DOCKER) {
   chromeFlags.push('--no-sandbox');
 
   // Enable debug logging from Chrome to help track down a cause of frequent
@@ -190,7 +189,7 @@ module.exports = function (config) {
     // start these browsers
     // available browser launchers: https://npmjs.org/browse/keyword/karma-launcher
     browsers: ['ChromeHeadless_Custom'],
-    browserNoActivityTimeout: 20000, // Travis is slow...
+    browserNoActivityTimeout: 20000,
 
     customLaunchers: {
       ChromeHeadless_Custom: {