Update datepicker library to a newer version

[daattali]

I can't figure out what version of datepicker shiny is using, but based on the fact that the javascript file hasn't changed since Dec 2014 I think it's at least as old as that. There are some new functionality that have been implemented since then that can be useful, for example in Jan 2015 they added a way to clear the date which I can't figure out how to do with the current datepicker.

As a sidenote, it'd be nice to have a list somewhere that says what version each library within shiny is using