Improve CI config: Add 'disable pre-dexing' gradle option

Add to Travis CI configuration the previous added feature to main project build.gradle
'Improve Build Server performance adding disable pre-dexing gradle option'.

Disable it only on your build server using ./gradlew clean assembleDebug -PdisablePreDex
http://tools.android.com/tech-docs/new-build-system/tips#TOC-Improving-Build-Server-performance.

Add another target to compare results and create script section using this option.
Add ARGS='-PdisablePreDex' to this target (can be extended to multiple arguments).

This change overrides default `gradlew build connectedCheck` command
and avoid possible issues due to depend on default behaviour and changes.

Documentation is not updated and I'm not familiar with the code but seems they use
Gradle v2.0 and other options when no gradle wrapper found to build and resolve dependencies.

Next step will be add to the PATH our GRADLE_USER_HOME location and other customizations.