Change jQuery to $ in function declaration

When compiled, `window.jQuery` is passed in and is set to the variable `$` internally. We should maintain that variable when defining the typeahead function.

https://github.com/twitter/typeahead.js/blob/d5402b3b62c3659a9107c014ba56322ff17ab170/dist/typeahead.jquery.js#L1115

https://github.com/twitter/typeahead.js/blob/d5402b3b62c3659a9107c014ba56322ff17ab170/dist/typeahead.jquery.js#L7