Update pytest requirement.
Update pytest requirement.

This breaks Travis.

    pkg_resources.VersionConflict: (pytest 2.6.3 (/home/travis/virtualenv/python2.7.8/lib/python2.7/site-packages), Requirement.parse('pytest==2.6.1'))
tempbottle/gunicorn,mvaled/gunicorn,malept/gunicorn,beni55/gunicorn,gtrdotmcs/gunicorn,1stvamp/gunicorn,jamesblunt/gunicorn,GitHublong/gunicorn,prezi/gunicorn,keakon/gunicorn,ccl0326/gunicorn,zhoucen/gunicorn,WSDC-NITWarangal/gunicorn,MrKiven/gunicorn,harrisonfeng/gunicorn,ephes/gunicorn,ccl0326/gunicorn,jamesblunt/gunicorn,mvaled/gunicorn,tejasmanohar/gunicorn,malept/gunicorn,elelianghh/gunicorn,zhoucen/gunicorn,z-fork/gunicorn,prezi/gunicorn,prezi/gunicorn,gtrdotmcs/gunicorn,gtrdotmcs/gunicorn,mvaled/gunicorn,zhoucen/gunicorn,ccl0326/gunicorn,malept/gunicorn,jamesblunt/gunicorn,1stvamp/gunicorn,1stvamp/gunicorn