Bump the redis-rails gem

Before this change, this app was running with a patched version
of the redis-activesupport gem which fixed a pretty nasty bug (
https://github.com/redis-store/redis-store/pull/166). This bug
was fixed upstream (https://github.com/redis-store/redis-store/pull/190)
and released with redis-activesupport 3.2.4 so we don't need to
maintain a separate branch any more.
Merge pull request #118 from alphagov/update-redis-store-gem

Bump the redis-rails gem