Cache the installed bundle on Travis

This is for #44.

Unfortunately, it doesn't build on Travis because the Gemfile.lock is committed (which is necessary for this bundle caching scheme).

To demonstrate, [here's a build on Travis ](https://travis-ci.org/promptworks/stacker_bee/jobs/22860442) that fails because the Gemfile.lock is committed ([here's the commit](https://github.com/promptworks/stacker_bee/commit/202e407cdc3ebb6727ffc2f369136dce18e488da)). It bundles locally just fine.

Any ideas? @justincampbell ?