CI: switch from Travis to Actions

They're actively maintained, faster on Mac and Windows, and generally
a better mechanism long-term.

The only change applied here, in practice, is that we now test on Mac
and Windows as well.

.github/workflows/test.yml

+on: [push, pull_request]
+name: Test
+jobs:
+  test:
+    strategy:
+      matrix:
+        go-version: [1.14.x, 1.15.x]
+        # TODO(mvdan): fix and enable windows
+        # os: [ubuntu-latest, macos-latest, windows-latest]
+        os: [ubuntu-latest, macos-latest]
+    runs-on: ${{ matrix.os }}
+    steps:
+    - name: Install Go
+      uses: actions/setup-go@v2
+      with:
+        go-version: ${{ matrix.go-version }}
+    - name: Checkout code
+      uses: actions/checkout@v2
+    - name: Test
+      run: go test ./...

.travis.yml

-language: go
-
-go:
-- 1.14.x
-- 1.15.x
-
-notifications:
-  email: false
-
-script:
-- go test ./...