Add zip adapter

This is an adaption layer that allows Warbler to support both the 0.9 and 1.0 APIs from RubyZip.

I'm not happy about the travis.yml changes, but I couldn't think of another way to prevent 1.x from being installed in those test environments. If nothing else, it's a reflection of the ugliness that an end-user has to deal with. :smirk: