Sampler: Bug Fix: Return a Promise instead of an Object
Sampler: Bug Fix: Return a Promise instead of an Object

Change Propagation's rule execution mechanism expects a Promise to be
returned when executing a rule. Alas, if the sampler is active, only an
Object is being returned. This commit fixes the obvious error.

This is a follow-up on PR #166

d00rman/restbase-mod-queue-kafka,Pchelolo/change-propagation,wikimedia/change-propagation,wikimedia/change-propagation,Pchelolo/change-propagation