Update build tools and API levels. Fixes T1016

Summary:
 - Update targetSdkVersion to 21
 - Update compiledSdkVersion to 21
 - Update support libraries to 21
 - Update travis config file to use build-tools 21.0.2
 - Update travis to use android-21

Test Plan:  - ./gradlew build

Reviewers: dkobia, PapaLee

Subscribers: shadowhand, rjmackay

Maniphest Tasks: T1016

Differential Revision: https://phabricator.ushahidi.com/D465