Roll back to leaflet 1.3.1: markercluster plugin broken

[DonNicoJs]

Apparently leaflet 1.3.4 breaks markercluster plugin.
This is the result of a quick test that I did.
If anyone wants to test / chime in is welcomed.
I will test this more in the coming days @jperelli maybe you are interested

[jperelli]

Is there an upstream issue? I didn't find it in leaflet-markercluster repository

[DonNicoJs]

@jperelli I did not check but with leaflet 1.3.4 there is an error in the console about the markercluster constructor
Goes away by rolling back to 1.3.1.

[DonNicoJs]

related to #281