Dependency on Bundler not stated

[strzibny]

gemnasium-parser depends on Bundler::Dependency, but the dependency is not stated in .gemfile. Could we delete the dependency on Bundler altogether and use Gem::Dependency instead? Given the fact that Gem::Dependency comes with RubyGems and that the Bundler features are slowly moving to RubyGems it would make sense to me. Otherwise/in the meantime let's fix this by stating the dependency on Bundler.