Switch to `redmine_acts_as_taggable_on`

Instead of requiring `acts-as-taggable-on` directly, use the
`redmine_acts_as_taggable_on` gem to take care of the "tags" and
"taggings" tables intelligently.
update redmine_acts_as_taggable_on

now ~> 0.2.0
don't mention acts-as-taggable-on requirement
update redmine_acts_as_taggable_on: ~> 0.3.0
update redmine_acts_as_taggable_on
Merge pull request #53 from ixti/use-redmine-aato-gem

Switch to `redmine_acts_as_taggable_on`