support jquery 2.0.x

Apparently https://github.com/jtsage/jquery-mobile-datebox/pull/269 wasn't enough. Something apparently slipped through my testing...

From jquery 2.0.0 ```$.mobile.activePage.jqmData('mobile-page')``` returns undefined.
Merge pull request #284 from jnyrup/patch-2

support jquery 2.0.x