Move CI to GitHub Actions (#7)

Author: samtstern

.github/workflows/java.yml

@@ -0,0 +1,17 @@
+name: Java CI
+
+on:
+  - pull_request
+  - push
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - name: set up JDK 1.8
+      uses: actions/setup-java@v1
+      with:
+        java-version: 1.8
+    - name: Build with Gradle
+      run: ./gradlew clean build

.travis.yml

@@ -8,4 +8,7 @@ on [firebase.google.com](https://firebase.google.com/docs/).
 We love contributions! See [CONTRIBUTING.md](./CONTRIBUTING.md) for guidelines.
 
 ## Build Status
-[![Build Status](https://travis-ci.org/firebase/snippets-java.svg?branch=master)](https://travis-ci.org/firebase/snippets-java)
+[![Actions Status][gh-actions-badge]][gh-actions]
+
+[gh-actions]: https://github.com/firebase/snippets-java/actions
+[gh-actions-badge]: https://github.com/firebase/snippets-java/workflows/Java%20CI/badge.svg