Enable github action cache for CI

.github/workflows/gradle.yml

@@ -1,4 +1,4 @@
-name: Java CI
+name: Halo CI
 
 on:
   pull_request:
@@ -15,6 +15,14 @@ jobs:
         uses: actions/setup-java@v1
         with:
           java-version: 1.8
+      - name: Cache Gradle
+        id: cache-gradle
+        uses: actions/cache@v1
+        with:
+          path: ~/.gradle/caches
+          key: ${{ runner.os }}-gradle-${{ hashFiles('**/*.gradle') }}
+          restore-keys: |
+            ${{ runner.os }}-gradle-
       - name: Test with Gradle
         run: ./gradlew test
       - name: Build with Gradle