Some clean up in build/docs

(a) Deleted an outdated line from the docs
(b) Removed a work around that is no longer necessary given the mesos version bump.

Author: Patrick Wendell <pwendell@gmail.com>

Closes #382 from pwendell/maven-clean and squashes the following commits:

f0447fa [Patrick Wendell] Minor doc clean-up
(cherry picked from commit 98225a6effd077a1b97c7e485d45ffd89b2c5b7f)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>