Upgrade mesos to 0.20.1

See https://github.com/apache/mesos/blob/master/CHANGELOG

Testing Done:
./gradlew -Pq build
./pants src/test/python:all
./src/test/sh/org/apache/aurora/e2e/test_end_to_end.sh

Bugs closed: AURORA-875

Reviewed at https://reviews.apache.org/r/26991/