p5-graphviz: update to version 2.18.

[dbevans (David B. Evans)]

Looks like some renewed interest in this module upstream. Latest changes:

Revision history for Perl module GraphViz.

2.18  Thu May 28 08:43:00 2015
	- All patches to this version are from chrony, via a github pull request. With thanx.
	- Use File::Which on Windows to detect if Graphviz is present.
	- Clean up the code, including Build.PL and Makefile.PL.

2.17  Mon Mar 30 08:56;00 2015
	- No code changes.
	- Create github repo. This has been done because ribasushi (Peter Rabbitson) has kindly offered
		some patches. Hence I've reversed my policy of encouraging this module to die :-).
	- Update Build.PL and Makefile.PL to point to the repo. None of the pre-reqs were updated.

[ryandesign (Ryan Carsten Schmidt)]

r137003