Fix get_graphviz_version()

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.