Upgrade the embedded version of pickadate.js

In going from v3.2.2 to v3.5.4 we gain support for the "hiddenName" picker option; this way, we can instantiate the widget as in
``` python
w = deform.widget.DateInputWidget(options={
                              'format': 'dd/mm/yyyy',
                              'selectYears': 10,
                              'selectMonths': True,
                              'formatSubmit': 'yyyy-mm-dd',
                              'hiddenName': True,
                             }
       )
```
This way, the widget can push its contents with the correct name even if the submit format is different from the display format.