Report proper gnuplot version.

If a custom version of gnuplot would be used by setting the GNUPLOT
environment variable, then still the default gnuplot was called to fill
in the gnuplot version number.

Signed-off-by: Sven van Haastregt <svhaastr@liacs.nl>

gitstats

 	return getpipeoutput(['git --version']).split('\n')[0]
 
 def getgnuplotversion():
-	return getpipeoutput(['gnuplot --version']).split('\n')[0]
+	return getpipeoutput(['%s --version' % gnuplot_cmd]).split('\n')[0]
 
 class DataCollector:
 	"""Manages data collection from a revision control repository."""