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.
bundler and rubygems and native gem compilation location have been getting kind of stupid lately, we definitely want to keep it frozen for chef-10 and chef-11 for now, and would need to do manual verification testing that existing installs that have native chef_gem installs would still be usable after an upgrade.  i think that is mostly on the rubygems side, but bundler is part of that ecosystem.