Removed spring from Gemfile, moved poltergeist and quiet_assets to groups

Spring still works just fine for me if I put it in `.gemfile` which
is a file that we have for installing gems according to each developer's
preference.  If I completely uninstall Spring from my system then it
still works to invoke every command that asks for Spring due to
Spring rescuring from `LoadError`.

If I then place Spring back in `.gemfile` it will figure out that it
is now present and will load, speeding up subsequent invocations.