Convert CI from Travis to GitHub Actions.

The performance of GitHub Actions is faster and more consistent than the free
tier of Travis CI. Build, lint, tests, and staticcheck for the same versions of
Go using Actions instead.

.github/workflows/go-presubmit.yml

@@ -0,0 +1,23 @@
+name: Go presubmit
+
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+    types: [opened, reopened, synchronize]
+
+jobs:
+  build:
+    name: Go presubmit
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        go-version: ['1.11', '1.12', '1.13', '1.14', '1.15']
+    steps:
+    - name: Install Go ${{ matrix.go-version }}
+      uses: actions/setup-go@v1
+      with:
+        go-version: ${{ matrix.go-version }}
+    - uses: actions/checkout@v2
+    - uses: creachadair/go-presubmit-action@default

.travis.yml

@@ -1,7 +0,0 @@
-language: go
-go:
-  - 1.11
-  - 1.12
-  - 1.13
-  - 1.14
-  - 1.15