Remove "from __future__ import print"

This was well-intentioned, but it causes more trouble
than it's worth.

Fixes #652

examples/multiping.py

@@ -8,7 +8,6 @@ multiple hosts and monitor their output interactively for a period=
 of time.
 """
 
-from __future__ import print_function
 
 from mininet.net import Mininet
 from mininet.node import Node