bug : current rrule link is dead, use CDN instead

**Description**  
current rrule link is dead, use CDN instead

**How to reproduce**  
On a fresh sylius install, import the plugin, if you go to "Lieux Click 'N' Collect", fields doesn't work correctly because the rrule package is not loaded correctly.

**Possible Solution**  
Use CDN : https://cdn.jsdelivr.net/npm/rrule@2.8.1/dist/es5/rrule.min.js

