Update Makefile to support latest (working) less compiler dependency

Makefile

@@ -43,7 +43,7 @@ $(CSS_OBJ): www/less/search.less $(LESSC) $(LESSHAT) $(WDN_MIXINS)
 	$(LESSC) --clean-css $< $@
 	
 $(LESSC):
-	npm install less
+	npm install less less-plugin-clean-css
 
 $(LESS_LIB)/%.less:
 	@mkdir -p $(@D)