Upgrade to Rails 4.1 and Mongoid 4

This is the result of merging the output of `rails-api new content-store
--skip-active-record --skip-sprockets --skip-javascript
--skip-test-unit`
with all the changes we'd made to the app so far.
Update mongoid config for mongoid 4
Fix time accuracy issues in test.

`Time#iso8601` includes miliseconds, whereas `Time#as_json` doesn't,
which was causing these tests to fail.

It was also necessary to reload the item from the db to avoid a rounding
error due to a difference in precision between Ruby and Mongo.
Upgrade airbrake.

Makes the workaround in the placeholder initializer unnecessary.
Merge pull request #34 from alphagov/upgrade_the_things

Upgrade the things (Rails 4.1 and Mongoid 4)