Don't try to embed links to Gists without username.

It causes unwanted title/embedding for links to the list of a
user's Gists, which is much more common.

src/Linkification/Embedding.coffee

@@ -187,7 +187,7 @@ Embedding =
         text: (_) -> _.title
     ,
       key: 'Gist'
-      regExp: /^\w+:\/\/gist\.github\.com\/(?:[\w\-]+\/)?(\w+)/
+      regExp: /^\w+:\/\/gist\.github\.com\/[\w\-]+\/(\w+)/
       el: (a) ->
         el = $.el 'iframe'
         el.setAttribute 'sandbox', 'allow-scripts'