Update the tox coverage environment to python2.7.

Python 2.6 uses a "hack" to call __complex__ and so we lost coverage of that. 2.7 does it correctly. See https://github.com/zopefoundation/zope.proxy/pull/6#issuecomment-103618850

tox.ini

@@ -27,7 +27,7 @@ commands =
 
 [testenv:coverage]
 basepython =
-    python2.6
+    python2.7
 commands = 
 #   The installed version messes up nose's test discovery / coverage reporting
 #   So, we uninstall that from the environment, and then install the editable