Will/fix rake test

Removes cleaning of reports directory.

This fixes an error on Mac OS X in which xargs was being passed an unsupported argument.
It also fixes an issue in Jenkins in which `rake test` reports were being deleted.

This means that we will need to run `rake clobber` before collecting coverage info to prevent old results from being used.

@jzoldak can you please review?  This needs to be merged ASAP to fix the rake test command.