Remove Git dependency from bundler

Proposal which fixes #1043 

This method is draws from how the [hoe](https://rubygems.org/gems/hoe) gem handles which files should be included in the released gem. @zenspider provides a great writeup on this on the [hoe homepage](http://www.zenspider.com/projects/hoe.html#manifesttxt).

The approach works particularly well with bundler's `rake release` since if the manifest changes the release will be blocked because of uncommitted changes. I've been using version of it on [knife-solo](https://github.com/matschaffer/knife-solo) for the past couple releases and it has served me quite well.