Upgrade to mesos 0.20.x

mesos 0.20.0 introduced a python binding change from mesos 0.19.0, refer to
http://mesos.apache.org/documentation/latest/upgrades/ for more details.

Signed-off-by: Chengwei Yang <yangchengwei@qiyi.com>
Merge pull request #2 from chengweiv5/master

Upgrade to mesos 0.20.x