Update jquery.timepicker.js to version 1.3.8.

Hi, the file `vendor/assets/javascripts/jquery.timepicker.js` had not been correctly updated to the code present in https://github.com/jonthornton/jquery-timepicker/blob/1.3.8/jquery.timepicker.js. I was suffering from [this error](https://github.com/jonthornton/jquery-timepicker/pull/246), so I updated it.