Fix ecto.migrate task

This fixes the migrator mix task `mix ecto.migrate Repo`

migrated_versions/1 no longer returns a tuple pair `{:ok, _}` since changes in https://github.com/elixir-lang/ecto/commit/6c604801af06fb683bfc9c26be921172b3230e32 make adapter methods raise exceptions rather than returning tuple pairs `{:error, _reason}` and `{:ok, _result}`