Standardise gem file on single quotes
Bump rack to 1.5.2
Bump json gem to latest version
Update to latest activesupport (3.2.12)

Our use of activesupport is minimal (and should be reviewed at some point) but we
might as well keep it up to date.
Pin gem versions for all non-dev/test dependencies

As per our style guide
Upgrade to latest sinatra

In the interests of having core dependencies up to date
Remove i18n gem

I can't see any use of this and tests still pass without it.
Merge pull request #56 from alphagov/update-rack-and-json

Tidy up dependencies