doc: update sphinx theme name

Since 1.3b1 release, theme name is changed to 'classic'.

See http://sphinx-doc.org/changes.html#release-1-3b3-released-feb-24-2015

doc/themes/mroonga/layout.html

@@ -7,7 +7,7 @@
     :copyright: Copyright 2011-2012 by the mroonga project, see AUTHORS.
     :license: LGPLv2, see LICENSE for details.
 #}
-{% extends "default/layout.html" %}
+{% extends "classic/layout.html" %}
 {% set favicon = "favicon.ico" %}
 
 {% block header %}