Use existing supervisor type instead of pid for Supervised Tasks

[devstopfix]

Removes Dialyzer warning:

The call
 'Elixir.XXX.Task.Supervisor':async_nolink('Elixir.XXX.Supervisor',_f@1::fun(() -> any()),[{'retries', 3}])

breaks the contract
  (pid(),'Elixir.GenRetry':retryable_fun(),'Elixir.GenRetry':options()) -> #{'__struct__':='Elixir.Task','owner':=term(),'pid':=term(),'ref':=term()}

Drop Elixir 1.7 as type is missing during release of 1.6 and 1.7