Accepted mime type fixes

According to various blog posts of the official github developer
website [1] [2], the mime preview for Releases and Search are no longer
needed.
Moreover, they recommend to use `application/vnd.github.v3+json` for
every requests.

[1]
http://developer.github.com/changes/2013-11-04-releases-api-is-official/
[2]
http://developer.github.com/changes/2013-10-29-search-api-becomes-an-official-part-of-github-api-v3/