Upgrade flutter_map v4

[HugoHeneault]

Hello,

flutter_map just got it's v4. https://pub.dev/packages/flutter_map/changelog

Could you update your dep so it works with it?

I hope their new TileLayer implem won't make it tough to migrate 🤞

Thanks :)

[HugoHeneault]

Just saw #136, will try it :)

[greensopinion]

this is now merged