fix markdown util path

without this fix an app will bring an error:

```bash
MacBook-Pro-Dmitri:Lico dmitri$ luvit server.lua
/usr/local/Cellar/luvit/0.8.2/lib/luvit/luvit.lua:309: /usr/local/Cellar/luvit/0.8.2/lib/luvit/module.lua:193: Failed to find module 'markdown'
	no field package.preload['markdown']
	Cannot find module /usr/local/Cellar/luvit/0.8.2/lib/luvit/markdown
	Cannot find module /Users/dmitri/github/Lico/modules/utils/modules/markdown
	Cannot find module /Users/dmitri/github/Lico/modules/utils/node_modules/markdown
	Cannot find module /Users/dmitri/github/Lico/modules/modules/markdown
	Cannot find module /Users/dmitri/github/Lico/modules/node_modules/markdown
	Cannot find module /Users/dmitri/github/Lico/modules/markdown
	Cannot find module /Users/dmitri/github/Lico/node_modules/markdown
	Cannot find module /Users/dmitri/github/modules/markdown
	Cannot find module /Users/dmitri/github/node_modules/markdown
	Cannot find module /Users/dmitri/modules/markdown
	Cannot find module /Users/dmitri/node_modules/markdown
	Cannot find module /Users/modules/markdown
	Cannot find module /Users/node_modules/markdown
```