replace {:ok} with the more standard :ok

[Th3-M4jor]

DO NOT MERGE THIS before we consider 0.10 to be in a stable state, as this is intended to be part of a future 1.0.0 release.

Rationale behind replacement: In all Elixir and Erlang libraries that I can think of, you don't see the one element tuple of {:ok} as a possible return value. Instead you usually see :ok | {:error, reason} when the happy path has no meaningful return value.

This has the added benefit of a micro-optimization that avoids allocating a one element tuple on the heap.

[jchristgit]

I am interested in merging this in a minor pre-1.0 release, although possibly after 0.10 since that will bring some basic support for running multi-node.