Use GitHub actions as a CI instead of Travis.

.github/workflows/ci.yml

@@ -0,0 +1,30 @@
+name: CI
+
+on: [push, pull_request]
+
+jobs:
+  build-and-test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: set up JDK 1.8
+        uses: actions/setup-java@v1.4.3
+        with:
+          java-version: 1.8
+
+      - name: Cache Gradle dependencies
+        uses: actions/cache@v2
+        with:
+          path: ~/.gradle/caches
+          key: ${{ runner.os }}-gradle-${{ hashFiles('**/*.gradle') }}
+          restore-keys: ${{ runner.os }}-gradle
+
+      - name: Build debug APK and run Tests
+        run: ./gradlew assembleDebug lintDebug testDebugUnitTest --stacktrace
+
+      - name: Upload APK
+        uses: actions/upload-artifact@v2
+        with:
+          name: app
+          path: app/build/outputs/apk/debug/*.apk