Add Travis CI config

This CL adds a continuous build config to the G2 repository. It will look like https://travis-ci.org/krasin/g2 but the address will be a bit different: https://travis-ci.org/synthetos/g2

One will have to activate Travis CI for g2 repository, preferably before merging this CL (to have a "green" build w/o waiting for the next commit). To activate it:

1. Go to https://travis-ci.org/profile
2. Press "sync now" button
3. Turn on Travis CI on G2 repository

After that, please, merge this CL.