ci: migrate to GitHub Actions

Due to the recent changes to the Travis CI platform (see [1]), we will
now use GitHub Actions to run the tests.

Reference: https://docs.github.com/en/free-pro-team@latest/actions/guides/building-and-testing-nodejs

[1]: https://blog.travis-ci.com/2020-11-02-travis-ci-new-billing

Backported from master: adffc03

.github/workflows/ci.yml

@@ -0,0 +1,43 @@
+name: CI
+
+on:
+  push:
+  pull_request:
+
+jobs:
+  test-node:
+    runs-on: ubuntu-latest
+
+    strategy:
+      matrix:
+        node-version: [8.x, 10.x]
+
+    steps:
+      - uses: actions/checkout@v2
+      - name: Use Node.js ${{ matrix.node-version }}
+        uses: actions/setup-node@v1
+        with:
+          node-version: ${{ matrix.node-version }}
+      - run: npm ci
+      - run: npm test
+        env:
+          CI: true
+
+  test-browser:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v2
+      - name: Use Node.js
+        uses: actions/setup-node@v1
+        with:
+          node-version: '10.x'
+      - run: npm ci
+      - run: npm test
+        env:
+          CI: true
+          BROWSERS: 1
+          NGROK_AUTH_TOKEN: ${{ secrets.NGROK_AUTH_TOKEN }}
+          SAUCE_USERNAME: ${{ secrets.SAUCE_USERNAME }}
+          SAUCE_ACCESS_KEY: ${{ secrets.SAUCE_ACCESS_KEY }}
+        timeout-minutes: 20

README.md

@@ -1,7 +1,7 @@
 
 # socket.io-client
 
-[![Build Status](https://secure.travis-ci.org/socketio/socket.io-client.svg?branch=master)](http://travis-ci.org/socketio/socket.io-client)
+[![Build Status](https://github.com/socketio/socket.io-client/workflows/CI/badge.svg)](https://github.com/socketio/socket.io-client/actions)
 [![Dependency Status](https://david-dm.org/socketio/socket.io-client.svg)](https://david-dm.org/socketio/socket.io-client)
 [![devDependency Status](https://david-dm.org/socketio/socket.io-client/dev-status.svg)](https://david-dm.org/socketio/socket.io-client#info=devDependencies)
 [![NPM version](https://badge.fury.io/js/socket.io-client.svg)](https://www.npmjs.com/package/socket.io-client)