MAHOUT-1364: Upgrade Mahout codebase to Lucene 4.6.

git-svn-id: https://svn.apache.org/repos/asf/mahout/trunk@1551935 13f79535-47bb-0310-9956-ffa450edef68
MAHOUT-1364: Upgrade Mahout codebase to Lucene 4.6, adding CHANGELOG entry for this.

git-svn-id: https://svn.apache.org/repos/asf/mahout/trunk@1551945 13f79535-47bb-0310-9956-ffa450edef68