[HOTFIX] Patching maven build after #6 (SPARK-1121).

That patch removed the Maven avro declaration but didn't remove the
actual dependency in core. /cc @scrapcodes

Author: Patrick Wendell <pwendell@gmail.com>

Closes #37 from pwendell/master and squashes the following commits:

0ef3008 [Patrick Wendell] [HOTFIX] Patching maven build after #6 (SPARK-1121).