jquery.makeCollapsible: Use promise().done instead of when().then

jquery.makeCollapsible: Use promise().done instead of when().then

Follows up b340bd4f6b.

They both work, but in this case there is no need to use $.when
since there is only 1 promise. And there is no need for the
filtering proxy of Promise#then either, a straight done is enough.

 $.when( $foo ).then( callback )
 $.when( $foo ).done( callback )
 $foo.promise().done( callback )

Change-Id: I4df755a32c124481ca690078191a735d2e2b511a