SPARK-1433: Upgrade Mesos dependency to 0.18.1

Enabled Mesos (0.18.1) dependency with shaded protobuf

Why is this needed?
Avoids any protobuf version collision between Mesos and any other
dependency in Spark e.g. Hadoop HDFS 2.2+ or 1.0.4.

ticket: https://issues.apache.org/jira/browse/SPARK-1433

Author      : berngp
Reviewer    : ?
Testing     : ?