Add TravisCI integration

We already run rspec via jenkins against the project before commits are
allowed through Gerrit, but TravisCI gives us a couple nice freebies:

  - Testing against multiple versions of Ruby. 1.8.7 compatibility is
    important for native OS X integration, and in general the more
    coverage the better.
  - Runs tests against pull requests (from third parties) before we even
    have jenkins run them. Defense in depths.
  - Shiny badges to show our compatibility.

Change-Id: I3715ab15324f0734c0e1b72b3344a953d4e0fe73
Reviewed-on: https://gerrit.causes.com/23500
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Joe Lencioni <joe.lencioni@causes.com>