build(deps): Bump github.com/golang/protobuf from 1.4.3 to 1.5.1

Bumps [github.com/golang/protobuf](https://github.com/golang/protobuf) from 1.4.3 to 1.5.1.
- [Release notes](https://github.com/golang/protobuf/releases)
- [Commits](golang/protobuf@v1.4.3...v1.5.1)

Signed-off-by: dependabot[bot] <support@github.com>

go.mod

@@ -14,7 +14,7 @@ require (
 	github.com/go-kit/kit v0.10.0
 	github.com/go-logfmt/logfmt v0.5.0
 	github.com/gogo/protobuf v1.3.2
-	github.com/golang/protobuf v1.4.3
+	github.com/golang/protobuf v1.5.1
 	github.com/gorilla/websocket v1.4.2
 	github.com/gtank/merlin v0.1.1
 	github.com/libp2p/go-buffer-pool v0.0.2