Make six an external dependency.
Add python-six as Travis dependency.
Install six from PyPi.  Travis version is too old.
Add rank filter tests as data dir
Fix 16bit rank filter test in python 3.x
Merge pull request #809 from ahojnnes/rank-setup

Fix rank filter tests
Update pip requirements.
Merge pull request #801 from stefanv/six_external

Make six an external dependency.