Move rake to the :test group
Move rake to the :test group

Rake extends Kernel with the #install method. So when one of our builders does not
override it (for example, PHP builder without Composer support), Rake's verison is used
and breaks the worker.

travis-ci/travis-worker,final-ci/travis-worker,final-ci/travis-worker,gavioto/travis-worker,ireznice/travis-worker,gavioto/travis-worker,travis-ci/travis-worker,ireznice/travis-worker