Update rubygems dependecies.

    $ bundle outdated   # Check outdated rubygems
    $ $EDITOR Gemfile
    $ bundle update     # Update installed gems in the Gemfile.lock
    $ bundle outdated   # Recheck outdated rubygems

Note: `bundle install` does not update installed gems in the Gemfile.lock.
See: http://gembundler.com/v1.3/man/bundle-update.1.html