Support Go Modules

[SamWhited]

Experiment with using Go Modules to build and bump Travis to the latest version of Go.

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "support_modules" [email protected]:SamWhited/libkv.git somewhere
$ cd somewhere
$ git commit --amend -s --no-edit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[SamWhited]

@thaJeztah this should also help simplify the libnetwork mod file. I had to fix CI as well which was broken but it should all be working again now.