Warning about `:elixir_errors`

[Ilia Borovitinov]

When compiling, the following warning is displayed:

==> pipe_to
Compiling 2 files (.ex)
warning: :elixir_errors.warn/3 is undefined or private
  lib/pipe_to.ex:39: PipeTo.~>/2

This issue is present since elixir 1.9, and is resolved by replacing it with IO.warn/2, like here: CrowdHailer/Ace#138