Use go install to set up promu

Author: chitoku-k

Makefile

@@ -35,7 +35,7 @@ test: deps
 promu:
 	@GOOS=$(shell uname -s | tr A-Z a-z) \
 		GOARCH=$(subst x86_64,amd64,$(patsubst i%86,386,$(shell uname -m))) \
-		$(GO) get -u github.com/prometheus/promu
+		$(GO) install github.com/prometheus/promu@v0.13.0
 
 build: promu
 	@echo ">> building binaries"