Ignore Android Studio related files

.idea as AS is based on IntelliJ, and .iml
Upgrade versions of gradle, gradle android plugin, and Android build tools
Update travis.yml build tools 19.1.0
Merge pull request #637 from garvankeeley/travis-update

Update travis.yml build tools 19.1.0
Merge pull request #633 from keverets/androidstudio

Android Studio tool update