collectstatic failes on various test suite files which are not needed to
install Mayan EDMS. Switch over to preparestatic, which contains a
predefined ignore list.
Upgrade Celery version used from 3.1.26 to 4.1.1. The following
settings have been renamed: CELERY_ALWAYS_EAGER to
CELERY_TASK_ALWAYS_EAGER, BROKER_URL to CELERY_BROKER_URL.
Signed-off-by: Roberto Rosario <Roberto.Rosario@mayan-edms.com>