Initial GitHub CI/CD

GitHub CI/CD is [a thing](https://github.com/features/actions)! We will attempt:

- CI: build + unit test
- replica tests: running `gh-ost` tests on multiple MySQL versions

.github/workflows/ci.yml

@@ -0,0 +1,20 @@
+name: CI
+
+on: [pull_request]
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@master
+
+    - name: Set up Go 1.12
+      uses: actions/setup-go@v1
+      with:
+        version: 1.12
+      id: go
+
+    - name: Build
+      run: script/cibuild