Fix PlotTest on windows

@ericbn: I was not able to reproduce the problem you have described in https://github.com/jenkinsci/plot-plugin/commit/3e79b7c618066c5f5a8cc09f597bfafdc212c571#commitcomment-7864751 presumably because all win boxes I have access to have cygwin preinstalled. Though, this fix works on linux and should be reasonably portable to work on windows too.