accept IO-like representation of Gemfile.lock

hey, with this patch it was easier to build a web interface on top of bundler-audit. i hope to re-release the web software as free software when i have the time, but i lost my copy for now. it can be licensed under the same terms and conditions as bundler-audit.
it might be a good idea to drop the "gem" method in favor of "rubygem", I think there is already `Kernel#gem` from rubygems.