Updated the doc for the hadoop-2.6 profile, which is new to Spark 1.4

Author: Taka Shinagawa <taka.epsilon@gmail.com>

Closes #6450 from mrt/docfix2 and squashes the following commits:

db1c43b [Taka Shinagawa] Updated the hadoop versions for hadoop-2.6 profile
323710e [Taka Shinagawa] The hadoop-2.6 profile is added to the Hadoop versions table