fetch ids without instantiating records using pluck

Not only is pluck more efficient for fetching ids, but instantiating a record with only an id can cause problems isfthere are after_find hooks that expect to have all the columns populated.  However, pluck was a rails 3.2 addition, so we have to test for it since this gem supports back to rails 3.0.0.
```
* [@twalpole Update for rails edge (4.2)](https://github.com/mbleigh/acts-as-taggable-on/pull/583)
```