Switching uuid4()[1:4] -> uuid4().hex[:4]

.hex is what we need to access to get at the ascii (hex) version
anyway.  Also, not sure why the previous version grabbed starting at
the index of 1... just grab the first characters instead.

mediagoblin/db/mixin.py

@@ -111,7 +111,7 @@ class MediaEntryMixin(object):
             # let's whack junk on there till it's unique.
             self.slug += '-'
             while check_media_slug_used(self.uploader, self.slug, self.id):
-                self.slug += uuid4()[1:4]
+                self.slug += uuid4().hex[:4]
 
     @property
     def description_html(self):