tox.ini: Remove py24

It's no longer supported because setup.py uses a context manager

--HG--
branch : tox_remove_py24_2
diff --git a/tox.ini b/tox.ini
index f095b79..11821c8 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,5 +1,5 @@
 [tox]
-envlist=py24,py25,py26,py27,py31,py32,py33,py34,pypy
+envlist=py25,py26,py27,py31,py32,py33,py34,pypy
 indexserver=
     default = http://pypi.python.org/simple
     testrun = http://pypi.testrun.org