Add go vet to build

Makefile

@@ -1,9 +1,10 @@
 .PONY: all build deps image lint test
+CHECK_FILES?=$$(go list ./... | grep -v /vendor/)
 
 help: ## Show this help.
 	@awk 'BEGIN {FS = ":.*?## "} /^[a-zA-Z_-]+:.*?## / {sub("\\\\n",sprintf("\n%22c"," "), $$2);printf "\033[36m%-20s\033[0m %s\n", $$1, $$2}' $(MAKEFILE_LIST)
 
-all: lint test build ## Run the tests and build the binary.
+all: lint vet test build ## Run the tests and build the binary.
 
 build: ## Build the binary.
 	go build -ldflags "-X github.com/netlify/gotrue/cmd.Version=`git rev-parse HEAD`"
@@ -16,8 +17,11 @@ image: ## Build the Docker image.
 	docker build .
 
 lint: ## Lint the code
-	golint `go list ./... | grep -v /vendor/`
+	golint $(CHECK_FILES)
+
+vet: # Vet the code
+	go vet $(CHECK_FILES)
 
 test: ## Run tests.
 	rm -f api/test.db
-	go test -v `go list ./... | grep -v /vendor/`
+	go test -v $(CHECK_FILES)