Sort versions by number

:fork_and_knife: 

This changes versions to be ordered according to [the semver spec](http://semver.org) instead of by `id`, as they were previously.  I used a semver extension for Postgres, which is located [here](https://github.com/theory/pg-semver).  I'm guessing we'll want to make some modifications to our cookbook to compile and use this new extension.  I'll admit I'm ignorant to what the ramifications are for Travis-CI.

Relevant Trello card:  https://trello.com/c/Pmq2x3VJ/139-sort-a-cookbook-s-versions-by-version-number