Adds gem zip to fix an error running rails server

The zip/zip dependency was causing a load error when trying to run rails server.  I narrowed down the dependency to airbrake.  Adding `s.add_runtime_dependency("zip")` fixes the problem.  The project is currently working by requiring my fork.

The error from my command line:

    /Users/tristansiegel/.rvm/gems/ruby-2.0.0-p451/gems/activesupport-4.0.3/lib/active_support/dependencies.rb:229:in `require': cannot load such file -- zip/zip (LoadError)