Fix support for platform dependent gems

Playing with the current version of geminabox I'm experiencing some problems uploading and handling platform dependent gems, e.g. libv8 [1].

Once a platform dependent gem is uploaded, the generated filename doesn't include platform information. That creates problems uploading the same gem for another platform and deleting the gem (in the end I had to remove the gem from the command line).

I have created a patch with the changes to make this work again. Please let me know if you need further information or any change.

[1] https://rubygems.org/gems/libv8