work with latest (1.4) golang

Makefile

@@ -20,7 +20,8 @@ $(WEAVER_EXE) $(WEAVEDNS_EXE):
 		rm $@; \
 		echo "\nYour go standard library was built without the 'netgo' build tag."; \
 		echo "To fix that, run"; \
-		echo "    sudo go install -a -tags netgo std"; \
+		echo "    sudo go clean -i net"; \
+		echo "    sudo go install -tags netgo std"; \
 		false; \
 	}