Enable github ci on pull request (#525)

.github/workflows/gradle.yml

@@ -1,6 +1,8 @@
 name: Java CI
 
-on: [push]
+on:
+  pull_request:
+  push:
 
 jobs:
   build:
@@ -8,10 +10,12 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v1
-    - name: Set up JDK 1.8
-      uses: actions/setup-java@v1
-      with:
-        java-version: 1.8
-    - name: Build with Gradle
-      run: ./gradlew build
+      - uses: actions/checkout@v1
+      - name: Set up JDK 1.8
+        uses: actions/setup-java@v1
+        with:
+          java-version: 1.8
+      - name: Test with Gradle
+        run: ./gradlew test
+      - name: Build with Gradle
+        run: ./gradlew build -x test