Makefile: update dependencies

Prevent build failures which may occur when dependencies is
not up to date by updating them with the go get -u flag.

Add the go get -f flag as well, to override the check that
each package has been checked out from the repo implied by
its import path.

Makefile

@@ -13,8 +13,8 @@ cov:
 
 deps:
 	@echo "--> Installing build dependencies"
-	@go get -d -v ./...
-	@echo $(DEPS) | xargs -n1 go get -d
+	@go get -d -f -u -v ./...
+	@echo $(DEPS) | xargs -n1 go get -d -f -u
 
 test: deps
 	./scripts/verify_no_uuid.sh