Replace hand-rolled lock code with `redis-lock` gem

This change means that there's less code to maintain, and it has a (slightly) nicer interface.

The lock wrapper code is pinched from the `transition` app.

As [suggested](https://github.com/alphagov/support-api/pull/8#issuecomment-52295422) by @jennyd on https://github.com/alphagov/support-api/pull/8.