Make mirror-gem Ruby 2.0 compatible

This adds checks before making Gem::SpecFinder calls to prevent errors while
using Ruby 2.0. Gem::SpecFetcher#fetch_with_errors has been replaced by
Gem::SpecFetcher#spec_for_dependency. Additionally, #spec_for_dependency
returns a Gem::Source object where #fetch_with_errors returned a String.

Fixes issue #75