jQuery to $ inside the plugin

Without this change, the following userscript fails:

```javascript
// ==UserScript==
// @name  Monkeys script dependency
// @match https://github.com/
// @require http://cdnjs.cloudflare.com/ajax/libs/jquery/2.1.1/jquery.js
// @require http://rawgit.com/brasofilo/repeatable-fields/master/repeatable-fields.js
// @grant none
// ==/UserScript==
start_up();
function start_up () {
    var FULL_table_html = '<table>etc</table>';
    $('.wrapper').prepend(FULL_table_html);
	$('.repeat').each(function() {
        $(this).repeatable_fields();
    });
}
```