Change Verk.Supervisor to support treamlined child specs

[Psli]

Elixir v1.5 allows child specifications, which specify how a child process is supervised, to be defined in modules.

children = [
  MyApp.Repo,
  MyApp.Endpoint,
  Verk.Supervisor
]

Supervisor.start_link(children, strategy: :one_for_one)

[edgurgel]

Thank you! I will release a new version soon!