- update version from 0.18.1 to 0.21.0
- I'm doing some tests in order to verify some spark jobs work fine on mesos 0.21.0 environment.

Author: Jongyoul Lee <jongyoul@gmail.com>

Closes #3934 from jongyoul/SPARK-3619 and squashes the following commits:

ab994fa [Jongyoul Lee] [SPARK-3619] Upgrade to Mesos 0.21 to work around MESOS-1688 - update version from 0.18.1 to 0.21.0