Fix mix format and format migrations too

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

Fix credo warnings

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

Show elixir version

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

Also lint migrations

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

Reset allow failure to false

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

priv/repo/migrations/20180702154630_remove_slug_for_event.exs

@@ -9,7 +9,7 @@ defmodule Mobilizon.Repo.Migrations.RemoveSlugForEvent do
 
   def down do
     alter table(:events) do
-      add :slug, :string, null: false
+      add(:slug, :string, null: false)
     end
   end
 end