Correct Bootstrap licensing info

See "New license" section of http://blog.getbootstrap.com/2014/01/30/bootstrap-3-1-0-released/
and https://github.com/twbs/bootstrap/blob/master/LICENSE

NOTICE

@@ -8,4 +8,5 @@ Distributed under `APLv2 <http://www.apache.org/licenses/LICENSE-2.0>`_.
 Bootstrap
 =================
 
-Distributed under `APLv2 <http://www.apache.org/licenses/LICENSE-2.0>`_.
+v3.1.0 and subsequent versions distributed under `MIT <http://opensource.org/licenses/MIT>`_.
+Versions prior to v3.1.0 distributed under `APLv2 <http://www.apache.org/licenses/LICENSE-2.0>`_.