Migration to update tagging_type

Previously we had the tags associated with 'KyuEntry' model, hence tagging_type was stored as 'KyuEntry' using polymorphic association. 
But recently we have renamed  'KyuEntry' model to 'Post', hence the tags were not picked with 'Post.tag_counts'.
Hence added migration to update tagging_type column of taggings table.

![taggings_table](https://cloud.githubusercontent.com/assets/1950768/3076343/0a28305c-e3d2-11e3-9468-7d39e151110c.png)