Add encoding needed for py2 sphinx build.

Got this error without it:

Configuration error:
There is a syntax error in your configuration file: Non-ASCII character
'\xe2' in file /home/travis/build/drvinceknight/Axelrod/axelrod/match.py
on line 161, but no encoding declared; see
http://python.org/dev/peps/pep-0263/ for details (match.py, line 161)
make: *** [html] Error 1

Author: drvinceknight

axelrod/match.py

@@ -1,3 +1,4 @@
+# -*- coding: utf-8 -*-
 from axelrod import Actions, Game
 from .deterministic_cache import DeterministicCache