protobuf: match protoc-gen-go to the protobuf version

The Go protoc plugin (protoc-gen-go) and the protobuf package itself
need to match versions. The previous Makefile installed the master
version of the plugin, but dep pinned protobuf itself to 1.1.0, and now
we're seeing problems on some developer machines where protobuf
complains about a mismatch.

This commit fixes the problem using an implementation from developer
discussion on

    golang/protobuf#763

and regenerates the protobuf using the new (old) plugin.

Fixes greenplum-db/gpupgrade#99 .

Gopkg.toml

@@ -24,6 +24,11 @@
 #   go-tests = true
 #   unused-packages = true
 
+# Tool dependencies. In general we want to control our dependencies via
+# Gopkg.lock instead of having `go get` pull in the latest master versions.
+required = [
+  "github.com/golang/protobuf/protoc-gen-go"
+]
 
 [[constraint]]
   name = "github.com/cloudfoundry/gosigar"

Makefile

@@ -30,7 +30,7 @@ depend:
 		dep ensure
 
 depend-dev: depend
-		go get -u github.com/golang/protobuf/protoc-gen-go
+		go install ./vendor/github.com/golang/protobuf/protoc-gen-go
 		go get golang.org/x/tools/cmd/goimports
 		go get github.com/golang/lint/golint
 		go get github.com/alecthomas/gometalinter