update to bundler 1.7.3

The `git_source` method was added in 1.6.0. We use this [in Supermarket](https://github.com/opscode/supermarket/blob/f569b1c7abee52ba3fce5e928557cdbc39de0fab/Gemfile#L4-L9) to make sure we pull GitHub repos from HTTPS.
Ok, I've already got the override working. I'm going to update this to just put a comment trying to explain why we're using the version we're using.