Move 'Distinct' out of select string and use .uniq instead

Fix issue https://github.com/mbleigh/acts-as-taggable-on/issues/357 

When using :any => true option in tagged_with method along with other ActiveRecord query get and error like: 

"PG::Error: ERROR: syntax error at or near "DISTINCT"

So i move 'Distinct' out to the end of tagged_with method definition and use .uniq method instead.