Fix for moonc -w

I get this traceback when I try to run `moonc -w`:

```
/usr/bin/lua5.2: /usr/local/lib/luarocks/rocks/moonscript/dev-1/bin/moonc:175: Missing sleep function; install LuaSocket
stack traceback:
	[C]: in function 'error'
	/usr/local/lib/luarocks/rocks/moonscript/dev-1/bin/moonc:175: in function 'get_sleep_func'
	/usr/local/lib/luarocks/rocks/moonscript/dev-1/bin/moonc:240: in function 'create_watcher'
	/usr/local/lib/luarocks/rocks/moonscript/dev-1/bin/moonc:267: in main chunk
	[C]: in ?
```

This tiny change fixes it. :smiley: