Fix SimpleCov Execution

Re: the discussion on 00d74fa53fe8ad3d6481457d4a64485a71c9fa06, I have modified the way SimpleCov works so that it only runs when specified. To generate coverage, run `rake test` with `COVERAGE=true`.

(I tried a rake task, but couldn't get it to work with SimpleCov.)

CC: @mojombo, @mattr-, @tombell.