Don't install graphviz>=0.6 under Python 2.6
Don't install graphviz>=0.6 under Python 2.6

Fixes failing build since graphviz dropped support for Python 2.6
Ref: https://github.com/xflr6/graphviz/pull/32

Safihre/cherrypy,cherrypy/cherrypy,cherrypy/cherrypy,Safihre/cherrypy