Remove usage of token from environment in Nostrum.Shard.Payload

[jchristgit]

Find a way to pass the token all the way down from the top application
supervisor (or the closest sane descendant) to the shard payload module, such
that we can eventually swap it out for a user-supplied value. The topmost
module used to pass it in should simply call Application.get_env itself such
that no user-visible change is seen.