Fix compatibility with Fedora 20

A fix for Python 2.7.5 on Fedora 20 changed the signature of
_execute_child to the signature of Python 2.7.6, breaking our version
check. This patch replace the now invalid version check with checking
the signature of the orignal function, and using the correct
replacement.
Merge pull request #1 from nirs/signature-check

Fix compatibility with Fedora 20