Fix crash in ribbonbar sample

The ArtProvider shouldn't be cloned in this case because SetArtProvider() does
not take ownership of the object in this case.  Because of that, once the
cloned provider goes out of scope and gets garbage collected, future uses will
crash.

Fixes #141.

samples/ribbon/ribbonbar_demo.py

@@ -335,7 +335,6 @@ class RibbonFrame(wx.Frame):
 
         if event.GetGalleryItem() != None:
             if provider == self._ribbon.GetArtProvider():
-                provider = provider.Clone()
                 gallery.SetArtProvider(provider)
 
             provider.SetColour(RB.RIBBON_ART_GALLERY_HOVER_BACKGROUND_COLOUR,