ci: Switch from Travis to GitHub Actions (#717)

- The Travis CI YAML specified Ubuntu Trusty as a runner OS, which is
  very end-of-life, so I switched those checks to GitHub Actions for
  consistency with the other jobs here (acceptance tests, etc).
- You can see a test workflow run at
  https://github.com/issyl0/terraform-provider-github/runs/2024800600.
- I didn't know if we wanted to do matrix builds to test multiple
  versions of Go, but that can be a goal for the future if we do. The
  development instructions for this provider still say Go 1.13.

Author: issyl0

.github/workflows/ci.yml

@@ -0,0 +1,18 @@
+name: GitHub Actions CI
+
+on: [push, pull_request]
+
+jobs:
+  ci:
+    runs-on: ubuntu-latest
+    env:
+      GOFLAGS: "-mod=vendor"
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions/setup-go@v2
+        with:
+          go-version: '1.13'
+      - run: make tools
+      - run: make lint
+      - run: make test
+      - run: make website-lint