Switch to new style Supervisor child spec

[nlopes]

Ensures the following warning disappears:

warning: Supervisor.Spec.worker/2 is deprecated. Use the new child specifications outlined in the Supervisor module instead
  lib/bigflake/application.ex:12: Bigflake.Application.start/2

[venkatd]

@stevedomin is this something that we would be able to get merged in? Trying to remove some deprecations in our app :)