Clean workspace before build/test. Because cleaning is always a good …

…approche, isn't it? ;)

.travis.yml

@@ -43,7 +43,7 @@ before_script:
   - adb shell input keyevent 82 &
 
 script:
-  - ./gradlew build jacocoTestReport assembleAndroidTest connectedCheck
+  - ./gradlew clean build jacocoTestReport assembleAndroidTest connectedCheck
 
 after_success:
   - bash <(curl -s https://codecov.io/bash)