Use GitHub Actions instead of Travis CI

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 slevithan#308
hasn't started in 13 minutes

.github/workflows/build.yml

@@ -0,0 +1,20 @@
+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
+    - 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'