add back the taggables index in the taggings table

Resolves issue https://github.com/mbleigh/acts-as-taggable-on/issues/509

As explained in the bug, taggings_idx can only be used if tag_id is specified in the where clause due to the nature of compound indexes. This means the following query requires a full table scan: "what are all the tags on this taggable?"

This a regression that is fixed by re-adding a compound index on [:taggable_id, :taggable_type, :context]