Improve function existance checking

You originally check on the gnome-shell version. However, Debian testing version of gnome-shell is 3.4.2, but seem's like that they backported something, because new Clutter.Color() is broken while Clutter.Color.from_string works. 
Instead, you can check about the existance of the from_string function in Color.
This should work for both older and newer systems.