This PR sets us up for managing `go-livepeer` dependencies using `go mod`.
Users building standalone lpms using `GOPATH` will still have to manually run `go get github.com/livepeer/joy4@livepeer` - This PR should at least get them a step closer by pointing them to the correct repository.
Users building standalone lpms using `go mod` should be automatically pointed to the correct revision of joy4.
Users building `go-livepeer` must continue building using the vendored lpms and `GOPATH` until `go mod` is completely ready.
This does nothing to harm to current state of things for users with today's build methods.
* add go.mod
* move from `github.com/nareix/joy4` to `github.com/livepeer/joy4@livepeer`
```
go get github.com/livepeer/joy4@livepeer
```
* go.mod: update dependency: github.com/livepeer/joy4 v0.1.1
dependabot[bot]