Use single quotes instead of double quotes

- This makes the airbrake gem line consistent with all of the others - I
  found this when looking through to upgrade Rails to v4.x and there was
  no reason for it to be quoted differently.
Merge pull request #321 from alphagov/use_single_quotes_in_gemfile

Use single quotes instead of double quotes
Update all of the gems so that Rails 4 works

- I've probably missed some, but there'll be further updates as I go
  through every error that now exists.