fix(build): replace dependencies on unstable gonum.org website

[sranka]

The CI builds fails to because of these errors:

go: gonum.org/v1/gonum@v0.0.0-20181121035319-3f7ecaa7e8ca: unrecognized import path "gonum.org/v1/gonum" (parse https://gonum.org/v1/gonum?go-get=1: no go-import meta tags ())

What was the problem?
The gonum.org web site does not serve go modules well, at least today.

Parsing meta tags from https://gonum.org/v1/netlib?go-get=1 (status code 404)

What was the solution?
Replace the dependency to use original sources at github.com

• Rebased/mergeable
• Tests pass
• Sign CLA (if not already signed)

[sranka]

I created gonum/gonum#1361 with a request to repair the gonum.org web site.

[kortschak]

This should be working again.

[sranka]

Thank you @kortschak for such a swift response, I confirm that it is now OK on our side. Closing, there is no need for a workaround.