go get specific

[adambrakhane]

It doesn't seem possible to specify the required version or commit of a package that an included package references. For example, github.com/gorilla/mux imports github.com/gorilla/context but adding a [deps.context] section to gopack.config leads to to an error: github.com/gorilla/context in gopack.config is unused.

The implication of this seems to be that we would always check out the HEAD of context every time it is requested.