Update transmission for python 2.7

Added recommandation from http://www.python.org/dev/peps/pep-0263/ to make this plugin work with pyhton 2.7.5+
Related error:
File "/etc/munin/plugins/transmission", line 36
SyntaxError: Non-ASCII character '\xc3' in file /etc/munin/plugins/transmission on line 37, but no encoding declared; see http://www.python.org/peps/pep-0263.html for details