Update Gemfile.lock for Heroku

To install it, we run bundle install with a special flag:

$ bundle install --without production

The --without production option prevents the local installation of any
production gems, which in this case consists of pg and rails_12factor.
(If Bundler complains about a readline error, try adding gem
'rb-read\-line', '~> 0.4.2' to your Gemfile.) Because the only gems
we’ve added are restricted to a production environment, right now this
command doesn’t actually install any additional local gems, but it’s
needed to update Gemfile.lock with the pg and rails_12factor gems and
the specific Ruby version. We can commit the resulting change as
follows:

$ git commit -a -m "Update Gemfile.lock for Heroku"