Switched `verify-reviews.py` to use ubuntu-16.04-based image.
Switched `verify-reviews.py` to use ubuntu-16.04-based image.

This not only uses a more recent platform to verify review, but also
works around MESOS-10091 which affects only older systems like e.g.,
ubuntu-14.04.

Review: https://reviews.apache.org/r/72054

reneploetz/mesos,abudnik/mesos,reneploetz/mesos,dforsyth/mesos,kaysoky/mesos,kaysoky/mesos,dforsyth/mesos,Gilbert88/mesos,abudnik/mesos,Gilbert88/mesos,Gilbert88/mesos,Gilbert88/mesos,dforsyth/mesos,kaysoky/mesos,reneploetz/mesos,dforsyth/mesos,abudnik/mesos,abudnik/mesos,Gilbert88/mesos,dforsyth/mesos,kaysoky/mesos,kaysoky/mesos,reneploetz/mesos,reneploetz/mesos,Gilbert88/mesos,abudnik/mesos,abudnik/mesos,reneploetz/mesos,dforsyth/mesos,abudnik/mesos,kaysoky/mesos,dforsyth/mesos,reneploetz/mesos,Gilbert88/mesos,kaysoky/mesos