Switch to Go modules for dependency management

[kytrinyx]

The vgo proposal has been accepted, I'd love to start using it sooner rather than later for the nextercism CLI:

golang/go#24301 (comment)

[nywilken]

@kytrinyx I'm changed the title of this issue to Switch to Go modules for dependency management as vgo is not officially a thing yet, but Go modules are for 1.11 which is the first step to vgo.