travis: add testing for go tip allowing failure
Created by: kortschak
Please take a look.
After suggestion for gonum/plot by @Kunde21 in https://github.com/gonum/plot/pull/349#issuecomment-297923393/
Created by: kortschak
Please take a look.
After suggestion for gonum/plot by @Kunde21 in https://github.com/gonum/plot/pull/349#issuecomment-297923393/