Switch to redmine_acts_as_taggable_on

Changes:

* Add redmine_acts_as_taggable_on to Gemfile
* Add the declaration (so that redmine_acts_as_taggable_on knows that
  this plugin needs the "tags" and "taggings" tables)
* Replace the initial migration with the one provided by
  redmine_acts_as_taggable_on
* Finally, remove the update_taggings_model migration. I've done this
  because it seems to be the best approach for all cases:
  * New users: it will always fail, so we might as well remove it
  * Existing users migrating up: the migration provided by
    redmine_acts_a_taggable_on creates the tables in the same way that
    the update_tagging_model migration would have changed them to, so we
    might as well remove it
  * Existing users migrating down: The table's being dropped, so it
    doesn't matter what we do -- but we should remove it so that it
    doesn't cause an error.
  (I've tested each of these cases myself)

I've only made the changes on master -- I'll let you decide which
branches to merge the changes into.