Move definition of "raven:test" rake task into .rake file.

We upgraded to the newest sentry gem today (0.12.2) and started running into the following issue:

As soon as the sentry gem is included in development, and more specifically, ```lib/raven/integrations/tasks.rb``` is loaded through the require in ```lib/raven/integrations/rake.rb```, we have our rspec test runner running after every rake task that is executed.

I assume this has something to do with using the Rake DSL out of place - I did not manage to find a bugfix, so instead I reworked the gem to use the normal rake task loading mechanism. This fixes our problem.

Note that this has the downside of having to explicitly require the Rake task in non-Rails environments.