Fix days difference in checkSelectionValid()

In function checkSelectionValid() (jquery.daterangepicker.js:1722) the days difference is counted as follows:

var days = Math.ceil((opt.end - opt.start) / 86400000) + 1;

which does not factor daylight saving time. Instead, it should be:

var days = countDays(opt.end, opt.start);

or using moment.js:

var days = moment(opt.end).diff(moment(opt.start), 'days');

[niutech]

Yes, I refer to this line. I think it is a good idea to use moment.js for date diffing, since it it well tested.

[holtkamp]

Probably resolved with https://github.com/longbill/jquery-date-range-picker/pull/423, released in https://github.com/longbill/jquery-date-range-picker/releases/tag/0.16.2