Nerves getting started guide is outdated

[axelson]

https://github.com/ScenicFramework/scenic/blob/391e9d2de4a19ec886f80d1873a0527eafd1b9cf/guides/getting_started_nerves.

mix scenic.new.nerves my_app no longer works. Instead https://github.com/ScenicFramework/scenic_new#build-a-basic-nerves-application should be followed