Bump github.com/golang/protobuf from 1.4.2 to 1.4.3

Bumps [github.com/golang/protobuf](https://github.com/golang/protobuf) from 1.4.2 to 1.4.3.
- [Release notes](https://github.com/golang/protobuf/releases)
- [Commits](golang/protobuf@v1.4.2...v1.4.3)

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

go.mod

@@ -6,7 +6,7 @@ require (
 	github.com/docker/docker v0.7.3-0.20190327010347-be7ac8be2ae0
 	github.com/go-openapi/spec v0.19.2
 	github.com/golang/glog v0.0.0-20160126235308-23def4e6c14b
-	github.com/golang/protobuf v1.4.2
+	github.com/golang/protobuf v1.4.3
 	github.com/mitchellh/go-homedir v1.1.0
 	github.com/prometheus/client_golang v1.0.0
 	github.com/prometheus/common v0.4.1