Remove error from mix task `triplex.migrations`

[PaulOstazeski]

Running mix triplex.migrations results in an error.

The stop callback for Ecto.Repo no longer takes a pid argument as of Ecto 3.

https://hexdocs.pm/ecto/2.2.12/Ecto.Repo.html#c:stop/2 https://hexdocs.pm/ecto/Ecto.Repo.html#c:stop/1

This update matches a change from #61 to the same effect.