Using 'yy' format would produce an error because getUTCYear() doesn't exist

- Fixed cause of error message: `Uncaught TypeError: Property 'getUTCYear' of object [object Date] is not a function`

It addresses open issue #159 (https://github.com/tarruda/bootstrap-datetimepicker/issues/159)

Sorry if there's a way to link to issues that I've not noticed; this is my first pull request.