Update oban to 0.8.1

This version uses a different locking mechanism, which gets rid of
`WARNING:  you don't own a lock of type ShareLock` log spam

priv/repo/migrations/20190917100019_update_oban.exs

@@ -0,0 +1,11 @@
+defmodule Pleroma.Repo.Migrations.UpdateOban do
+  use Ecto.Migration
+
+  def up do
+    Oban.Migrations.up(version: 4)
+  end
+
+  def down do
+    Oban.Migrations.down(version: 2)
+  end
+end