Switch to a public DNS accessible from GitHub

[tobiasehlert]

Supposed to be api.tibiadata.com in the long run, but can first be added when v3 is fully released.

https://github.com/TibiaData/tibiadata-api-assets/blob/de56532bafdad12c6be348e7d7b89e6f37ea828d/src/main.go#L21

The tibiadata/tibiadata-api-go#26 PR is needed to be in place as well.