update bundler to 1.7.2

:fork_and_knife: 

Currently supermarket-staging is running bundler 1.5.2.  https://github.com/opscode/supermarket/pull/792 introduced code using the `git_source` method, which wasn't added to bundler until 1.6.0, so CCR on supermarket-staging currently fails.

This fixes that by updating bundler to the latest stable version.