Enable Travis!

This adds the most basic Travis CI configuration, which will run `gradle assemble`. We can add more as necessary.

[Example build](https://travis-ci.org/rgbkrk/AccountsClient/builds/23444229).

![screen shot 2014-04-21 at 12 24 31 pm](https://cloud.githubusercontent.com/assets/836375/2756645/d7065f90-c979-11e3-9788-5fc028af6ef3.png)