[DOCS] [MINOR] Update for the Hadoop versions table with hadoop-2.6

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

docs/building-spark.md

@@ -80,6 +80,7 @@ Because HDFS is not protocol-compatible across versions, if you want to read fro
     <tr><td>2.2.x</td><td>hadoop-2.2</td></tr>
     <tr><td>2.3.x</td><td>hadoop-2.3</td></tr>
     <tr><td>2.4.x</td><td>hadoop-2.4</td></tr>
+    <tr><td>2.6.x and later 2.x</td><td>hadoop-2.6</td></tr>
   </tbody>
 </table>