Fix building when Gtk doesn't support version check

This is a quick patch to fix a build problem that I ran into this morning when updating matplotlib via pyston on my Ubuntu 12 laptop.  I haven't dug into this deeply, but this
is my current understanding (and the patch _does_ fix the build problem on my laptop):

python setup.py install --user
fails with
AttributeError: 'gi.repository.Gtk' object has no attribute 'get_major_version'

for old versions of Gtk without support for introspection
(specifically, on Ubuntu 12 with python-gtk2 2.24.0-3)

This commit silently drops the associated exceptions for the two Gtk
version checks in setupext.py