Fix needless rebuilding by touching ./local at end of build

Makefile

@@ -12,6 +12,7 @@ $(ENV): requirements.txt
 	$(VIRTUALENV) --no-site-packages $(ENV)
 	$(ENV)/bin/pip install -r requirements.txt
 	$(ENV)/bin/python ./setup.py develop
+	touch $(ENV)
 
 .PHONY: test
 test: | $(TOOLS)