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.
Merge pull request #289 from domcleal/rpm/develop-minitest

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