Ignoring Gemfile.lock

Ignoring platform-specific Gemfile.lock
Yes, this is not supposed to be how it works. But
https://github.com/bundler/bundler-features/issues/4 references a long
history of complaints and they are fixing this to make Gemfile.lock be
more cross-platform. If we don't ignore it, we'll be checking in this
file over and over again if we want to work on Windows, Mac, and
Linux.