Fix warning when using ecto 3.5.

[xward]

Fixed some warning that cause failure because missing embed_as implementation.

==> ecto_ulid
Compiling 1 file (.ex)
warning: function embed_as/1 required by behaviour Ecto.Type is not implemented (in module Ecto.ULID)
  lib/ecto/ulid.ex:1: Ecto.ULID (module)
warning: function equal?/2 required by behaviour Ecto.Type is not implemented (in module Ecto.ULID)
  lib/ecto/ulid.ex:1: Ecto.ULID (module)

did the same fix as seen here :adam12/ecto_network#17

[xward]

Travis doesn't like it :)

== Compilation error in file lib/ecto/ulid.ex ==
** (UndefinedFunctionError) function Ecto.Type.__using__/1 is undefined or private
    Ecto.Type.__using__([])
    lib/ecto/ulid.ex:6: (module)
    (stdlib) erl_eval.erl:680: :erl_eval.do_apply/6
The command "mix test" exited with 1.

What about doing an upgrade (recent elixir/ecto) release ?

[DanielDent]

This is no longer just a warning on Ecto 3.5 -- code that used to work is now broken.