Instructions to test-drive unreleased code

#42 
I can't think of any technical solutions. There doesn't seem to be any easy way to do this without writing a bunch of Ruby code.

Perhaps we should just explain to read the Gemfile.lock for the branch of CocoaPods you are trying to use and ensure the commits are locked to the same commits.

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Here's an example showing people opening issues for this [here](https://github.com/CocoaPods/CocoaPods/issues/2889#issuecomment-64991303) and [here](https://github.com/CocoaPods/CocoaPods/issues/2869). We've already had two support requests because of this kind of bundler setup advice.