Add travis config for people who want to contribute on GitHub

.travis.yml

+language: android
+
+jdk:
+  - oraclejdk8
+
+android:
+  components:
+    - platform-tools
+    - tools
+    - build-tools-25.0.2
+    - android-25
+    - extra-android-m2repository
+    - extra-google-m2repository
+
+script: "./gradlew assembleDebug"