Fix get_graphviz_version()'s regexp searching for a version number.  Because this used '*', any string matched the expression.  On my version of graphviz, 'dot -V' returned 'dot - graphviz version 2.30.1 (20130214.1330)', so get_graphviz_version() returned 'dot' as the version.
Merge pull request #2336 from embray/fix-get-graphviz-version

Fix get_graphviz_version()