"bootstrap_repos" task also tries to bootstrap "template" repos (like shared and pod-template)

`rake bootstrap_repos` searches for **every directory** in the Rainforest and tries to execute `rake bootstrap` on each `Rakefile` found.

This causes an issue because it **tries to run `rake` on "template" repos** (that are not actual code repos but host files to be used as templates) too, **like `CocoaPods/shared` and `CocoaPods/pod-template`**.

Shouldn't we bootstrap **only the repos listed in `GEM_REPOS`**?

/c @irrationalfab @alloy