Upgrade cucumber-rails and poltergeist to 1.4.0

Move specific limelight config out of env.rb and into limelight.rb,
because otherwise it will get overwritten with every gem update.
Upgrade uglifier, rspec-rails, brakeman
Upgrade GDS gems: slimmer, plek, frontend toolkit
Upgrade rails to 3.2.14, httparty, multi_xml

Because who needs Rails 4.0.0 in their life?
Merge pull request #193 from alphagov/the-big-gem-update-55980740

The big gem update