Setup Travis CI for google/physical-web addressing #68

This pull request addresses issue #68 

1. Created a new .travis.yml file containing the build for the android environment to build the project. This helps Travis CI system to recognize the project and the environment needed to build it. 
2. Setup a build script for running custom gradle tasks (currently running "assembleRelease" tasks, but more can be added, example "assembleDebugTest" etc.)
3. Update the contributors file (CLA has been submitted to Google). 

After the change is merged, we can update the "README" file with the following annotation to show the "build status" :
[![Build Status](https://travis-ci.org/google/physical-web.svg?branch=master)](https://travis-ci.org/google/physical-web)

Please review and let me know the feedback. 

Thanks!
Amit A