Cannot run ajax javascript code

I try to run the below javascript code but it does not work. (it does not return the alert pop up) I put the url string into a browser, it return the item info. So, I assume the url is fine. And the server return the information I want.

URL to brower:

/_api/web/lists/getbytitle('<Listname>')/items(1)/AttachmentFiles/

Returns:

<?xml version="1.0" encoding="utf-8"?><feed xml:base="myurl/_api/"; xmlns="w3.org/2005/Atom"; xmlns:d="schemas.microsoft.com/ado/2007/08/dataservices"; xmlns:m="schemas.microsoft.com/ado/2007/08/dataservices/metadata"; xmlns:georss="georss.org/georss"; xmlns:gml="opengis.net/gml">
    <content type="application/xml">
        <m:properties>
            <d:FileName>attached_filename.csv</d:FileNa‌​me>
            <d:ServerRelativeUrl>/DART/Lists/test/Attachments/1/attached_filename.csv</d:S‌​erverRelativeUrl>
        </m:properties>
    </content>
</entry></feed>

But why the code stop here? What is wrong? in the code? in the server? Any help is appreciated. Thank you so much!

$.ajax({
    type: "GET",
    url: url + "/_api/web/lists/getbytitle('" + name + "')/items(" + id + ")/AttachmentFiles/",
    headers: {
      "accept": "application/json;odata=verbose",
      "content-type": "application/json;odata=verbose"
    },
    success: function(data) { alert('succ');},
    error: function() {alert('err');}
  });

I am including the following before the script:

<script type="text\javascript" src="code.jquery.com/jquery-1.11.3.min.js">; </script>

After I looked into the error info, I found the $ is underfined error. Any idea why? How can I defined $? Thanks! SCRIPT5009: '$' is undefined it is near the ajax.