Stricter dialyzer options

Makefile

@@ -39,7 +39,7 @@ dialyzer: compile
 	@echo Use "'make build_plt'" to build PLT prior to using this target.
 	@echo
 	@sleep 1
-	dialyzer -Wno_return -Wunmatched_returns --plt $(COMBO_PLT) ebin | \
+	dialyzer -Wunmatched_returns --plt $(COMBO_PLT) ebin | \
 	    fgrep -v -f ./dialyzer.ignore-warnings
 
 cleanplt: