Cleanup migration warnings
From a clean install:
warning: unused alias Repo
priv/repo/migrations/20170719152213_add_follower_address_to_user.exs:5
warning: unused alias User
priv/repo/migrations/20170719152213_add_follower_address_to_user.exs:5
warning: unused import Ecto.Query
priv/repo/migrations/20170719152213_add_follower_address_to_user.exs:3
warning: unused import Supervisor.Spec
priv/repo/migrations/20170719152213_add_follower_address_to_user.exs:4