Version-peg protoc-gen-go

The provider module (github.com/golang/protobuf) was already in go.mod
as a build dependency.

.travis.yml

@@ -21,7 +21,6 @@ install:
   - cd ../harmony
   - go get -v ./...
   - go get gopkg.in/check.v1
-  - go get github.com/golang/protobuf/protoc-gen-go
   - scripts/install_build_tools.sh
 script:
   - go build -v ./...

tools/tools.go

@@ -11,6 +11,7 @@ package tools
 // scripts/install_build_tools.sh parses these imports to install them.
 import (
 	_ "github.com/golang/mock/mockgen"
+	_ "github.com/golang/protobuf/protoc-gen-go"
 	_ "github.com/harmony-ek/gencodec"
 	_ "golang.org/x/lint/golint"
 	_ "golang.org/x/tools/cmd/goimports"