Updated gemfile to new ffi, mime-types, rspec-core to avoid "ffi gem not found" error

I was getting the error "Could not find ffi-1.9.1 in any of the sources" when trying to bundle. Deleting the Gemfile.lock and re-bundling fixed it. Turns out ffi-1.9.1 has been yanked: http://rubygems.org/gems/ffi/versions and this was the version in the Gemfile.lock.