Add minitest dep as rubygem-activesupport 3.2.x is missing it

apipie-rails 0.2.x tries to reference ActionController::TestCase during init
which eventually requires minitest to be loaded.  4.x packages have the
explicit dependency.

Fixes failed builds (e.g. http://koji.katello.org/koji/taskinfo?taskID=132271) since https://github.com/theforeman/foreman/commit/6592ce8ab995428ead4037b2f4f753863fa5335b.

If you merge this, please click rebuild on this: http://ci.theforeman.org/job/release_nightly_build_rpm/103/