How do I get emmet to add an attribute with a value into a div tag?

The behaviour of Emmet-vim was changed to be as expected from documentation:

You don’t have to specify attribute values: td[colspan title] will produce <td colspan="" title=""> with tabstops inside each empty attribute (if your editor supports them).

So no. You can follow this request here: Attributes without values not being expanded.

Possible crude workaround could be to change the line 220 in autoload/emmet/lang/html.vim from

let current.attr[atts] = ''

to

let current.attr[atts] = function('emmet#types#true')