Remove captions temporarily due to Sphinx bug.

Captions on graphviz objects are broken in Sphinx 1.3.1. The bug has
been fixed but the fix hasn't been released yet, so we should avoid
using captions on graphviz graphs for the time being.

See sphinx-doc/sphinx#1788

docs/overview.rst

@@ -31,7 +31,6 @@ How are snippets retrieved by Firefox?
 --------------------------------------
 
 .. digraph:: snippet_download_flow
-   :caption: Flowchart illustrating the process of downloading a snippet.
 
    load_abouthome[label="User loads\nabout:home"];
    check_cache_timeout[label="Has it been\n4 hours since\nsnippets were fetched?" shape=diamond];