Generate .gems with rake task

[rylwin]

I created a rake task to generate the .gems file from gems listed in environment.rb. I thought this code (http://gist.github.com/433530) might be useful to include in the heroku gem.

[ddollar]

Hey there,

You may want to check out http://github.com/ddollar/heroku-manifest

Thanks for the submission as well, I'd suggest packaging up functionality like this in the future as a heroku gem plugin.