Stop ignoring Gemfile.lock so we can deploy to Heroku
Take out Windows and Linux notification gems

This is an attempt to make Heroku accept my Gemfile and Gemfile.lock. It had
been rejecting the push, I think because Gemfile mentions gems not listed in
Gemfile.lock (even though they are within system-identifying conditionals in
Gemfile). Perhaps in the future we can just include those gems regardless of
platform; or find a better way to dispatch conditioned upon platform.
Attempt #2 at eliminating system-conditional gem declarations

This time I just took out the conditional around the Mac-specific gems.