Add TravisCI integration

Add `.travis.yml` and the corresponding badge to the README file.

Currently supported Ruby versions are 1.9.3 and 2.0.0. The README was
updated from saying "1.9+" to "1.9.3+".

Change-Id: Ia98725947fd6618f050adfa7a1e162f939cbf585
Reviewed-on: https://gerrit.causes.com/27826
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Lann Martin <lann@causes.com>