[READY] Update rake task

### Problem

The travis build (https://travis-ci.org/BBC-News/alephant-publisher-request/builds/37830633) is currently failing because the specs are trying to use a notifier once they've run which isn't available on travis.

### Solution

The logic in the `Rakefile` has been extracted to the following gem: [rake-rspec](https://github.com/revett/rake-rspec) which leaves it up to the implementor to add formatting and how they're notified.