Fix 'Thread object has no attribute _children' error on my machine.
Patch to fix buggy assumption by multiprocessing library
Use system packages.

Install zmq from apt-get.
More experimenting with travis.
Updates to get Travis (mostly) working:

Patch to fix buggy assumption by multiprocessing library
Skip tests which require parakeet if it is unavailable.
Reduce logging for nosetests.
Use temporary directory instead of hdfs for testing.
Don't use apt for zmq; package is broken.