Upgrade dependency marshmallow to ==2.12.2 (#683)
Upgrade dependency marshmallow to ==2.12.2 (#683)


kevgliss/lemur,nevins-b/lemur,kevgliss/lemur,Netflix/lemur,Netflix/lemur,Netflix/lemur,Netflix/lemur,nevins-b/lemur,nevins-b/lemur,kevgliss/lemur