Unversioned python binary is not available anymore

As git_mulitmail python script is still python2 only per 2to3 script
set the binary to python2.

Author: prahal

Diff for: contrib/hooks/post-receive/lib/git_multimail.py

@@ -1,4 +1,4 @@
-#! /usr/bin/env python
+#! /usr/bin/env python2
 
 __version__ = '1.5.0'