Celery: Default changed from Msgpack to cpickle
Celery: Default changed from Msgpack to cpickle

* Change the way to serialize message to communicate with celery
  to serialize complex object we need to use cpickle instead of
  Msgpack

NOTE: beware, patch amended by Tibor to use new file names

tiborsimko/invenio-celery,tiborsimko/invenio-celery,inveniosoftware/invenio-celery,inveniosoftware/invenio-celery