Bump go to 1.12.4 / Switch to go modules (#564)

* Bump go to 1.12.4 / Switch to go modules

Follow-up for https://github.com/roboll/helmfile/pull/560#issuecomment-486516109

Makefile

@@ -1,12 +1,8 @@
 ORG     ?= $(shell basename $(realpath ..))
 PKGS    := $(shell go list ./... | grep -v /vendor/)
 
-dep: ## Get build dependencies
-	go get -v -u github.com/golang/dep/cmd/dep
-.PHONY: dep		
-
-build: ## Build the app after getting all dependencies (make sure to call make dep before)
-	dep ensure && go build ${TARGETS}
+build:
+	go build ${TARGETS}
 .PHONY: build
 
 generate:
@@ -18,7 +14,7 @@ fmt:
 .PHONY: fmt
 
 check:
-	dep ensure && go vet ${PKGS}
+	go vet ${PKGS}
 .PHONY: check
 
 test: