Fix KeyError when extensions with no version installed

https://bugzilla.gnome.org/show_bug.cgi?id=730177
PyGTKDeprecationWarning: Using positional arguments with the GObject constructor has been deprecated.

PyGTKDeprecationWarning: Using positional arguments with the GObject
constructor has been deprecated. Please specify keywords for label or
use a class specific constructor. See:
https://wiki.gnome.org/PyGObject/InitializerDeprecations

https://bugzilla.gnome.org/show_bug.cgi?id=725926