Update frank-cucumber.gemspec

Placate bundler when using Frank from a git source

Error was:
frank-cucumber at /Users/jtomson/.rvm/gems/ruby-1.9.3-p327@garuda/bundler/gems/Frank-938f76782b47/gem did not have a valid gemspec.
This prevents bundler from installing bins or native extensions, but that may not affect its functionality.
The validation message from Rubygems was:
  ["frank-skeleton/frank_static_resources.bundle"] are not files