Add support for packages with underscores

[nickjones]

gopkg considers this repo an unsupported URL pattern even though the github repo exists: http://gopkg.in/nickjones/proc_box.v1.

[GeertJohan]

An underscore in a package name!? https://blog.golang.org/package-names

[GodsBoss]

Prometheus Golang client is unobtainable, too: https://godoc.org/github.com/prometheus/client_golang

[Envek]

Also couldn't do go get gopkg.in/prometheus/client_golang.v0

[niemeyer]

The constraints on naming were always intentional. If a path is unobtainable, it means gopkg.in was being used as the canonical import path for the project, in which case it simply won't work well. The package authors should be on board with gopkg.in for it to be used.