Upgrade Mesos to 0.18.0.

Unblocks AURORA-14 from switching to an apache.org source for building
the Mesos .egg.

Testing Done:
Ran e2e test locally:

+ RETCODE=0
+ collect_result
+ [[ 0 = 0 ]]
+ echo '***'
***
+ echo 'OK (all tests passed)'
OK (all tests passed)
+ echo '***'
***
+ exit 0

Bugs closed: AURORA-322

Reviewed at https://reviews.apache.org/r/20744/