SPARK-1996. Remove use of special Maven repo for Akka

Just following up Matei's suggestion to remove the Akka repo references. Builds and the audit-release script appear OK.

Author: Sean Owen <sowen@cloudera.com>

Closes #1170 from srowen/SPARK-1996 and squashes the following commits:

5ca2930 [Sean Owen] Remove outdated Akka repository references