www/rubygem-redmine_acts_as_taggable_on: Update RUN_DEPENDS for rails4

This port is a dependency of redmine-knowledgebase. Redmine now depends
on rails4. Changing the dependency to the newer gem that uses rails4
enables this module to be buildable/installabale again.

Not going to MFH as 2016Q2 has the old redmine that uses rails3.