Add #ungem to DSL. 

The use case for this is when you use, for example, #gemspec but then have a dependency issue with a particular version of Ruby. This allows one to still use #gemspec, but exempt a requirement.

NOTE: The implementation is just an online "quick edit" designed by looking at #gem and figuring how it should probably work. I have not tested it. It might need to be refined as well, to take dependency type into account as I am not sure if it should or not.