Downgrade to Rails 3.2

So that we can use Mongoid (it doesn't support Rails 4 yet)
Fix put_json test helper to work with Rails 3.

Rails 3 integration stuff has some special handling for content-type,
but this is done before the case etc. is normalised, so if this doesn't
match the case that Rails uses internally, oddness ensues.
Switch to Mongoid
Drop and create database before running tests.

This ensures that any defined indexes have been created.
Clean up mongoid.yml, set safe to w: 1

Also remove redundant mongo.yml
Update seeds to work with mongoid
Merge pull request #37 from alphagov/mongoid

Switch to using Mongoid