IE8 Compatibility

[wellso]

When I click the th element to begin sorting on IE8 I get a problem on line 71;

User Agent: Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 6.1; Trident/4.0; SLCC2; .NET CLR 2.0.50727; .NET CLR 3.5.30729; .NET CLR 3.0.30729; Media Center PC 6.0; .NET4.0C; .NET4.0E)
Timestamp: Tue, 1 Mar 2016 11:21:30 UTC

Message: Object doesn't support this property or method
Line: 71
Char: 9
Code: 0
URI: http://192.168.10.10/assets/js/sortable.js

Think its possibly due to the way IE8 implements getAttribute, going to have a play around and see if I can fix it in a pull request.