Handle version string sent to stderr

An unfortunate side effect of switching from print to output() is
that all output() goes to stderr. We should probably carefully
consider whether this is the right thing to do.

util/versioncheck.py

@@ -4,7 +4,7 @@ from subprocess import check_output as co
 from sys import exit
 
 # Actually run bin/mn rather than importing via python path
-version = 'Mininet ' + co( 'PYTHONPATH=. bin/mn --version', shell=True )
+version = 'Mininet ' + co( 'PYTHONPATH=. bin/mn --version 2>&1', shell=True )
 version = version.strip()
 
 # Find all Mininet path references