Add .gitlab-ci.yml

.gitlab-ci.yml

+image: openjdk:8-jdk
+
+variables:
+  GRADLE_VERSION: "2.14.1"
+
+before_script:
+  - gradle wrapper --gradle-version ${GRADLE_VERSION} -Dcom.android.build.gradle.overrideVersionCheck=true
+
+stages:
+  - build
+
+build:
+  stage: build
+  script:
+    - ./gradlew assembleDebug
+  artifacts:
+    paths:
+    - app/build/outputs/
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