Fix type spec of Mailer.deliver_later/1 (#584)

What changed?
============

This commit updates the `Mailer.deliver_later/1` `@spec` for the
function found in `__using__`. It seems to not have been updated
with the release of Bamboo 2.0. 

Why?
-----

After updating to 2.0.0, dialyzer fails with a message like this:

lib/my_app/mailer/mailgun.ex:17:invalid_contract
The @SPEC for the function does not match the success typing of the function.

Function:
MyApp.Mailer.Mailgun.BambooMailer.deliver_later/1

Success typing:
@SPEC deliver_later(_) ::
  {:error,
   <<_::64, _::size(8)>>
   | %{
       :__exception__ => true,
       :__struct__ => Bamboo.EmptyFromAddressError | Bamboo.NilRecipientsError,
       :message => <<_::64, _::size(8)>>
     }}
  | {:ok,
     %{
       :bcc => {binary(), binary()},
       :cc => {binary(), binary()},
       :from => {binary(), binary()},
       :to => {binary(), binary()},
       _ => _
     }}

________________________________________________________________________________
done (warnings were emitted)

lib/bamboo/mailer.ex

@@ -80,7 +80,8 @@ defmodule Bamboo.Mailer do
  Bamboo.Mailer.deliver_now!(config.adapter, email, config, opts)
  end
 
- @spec deliver_later(Bamboo.Email.t()) :: Bamboo.Email.t()
+ @spec deliver_later(Bamboo.Email.t()) ::
+ {:ok, Bamboo.Email.t()} | {:error, Exception.t() | String.t()}
  def deliver_later(email, opts \\ []) do
  config = build_config(opts)
  Bamboo.Mailer.deliver_later(config.adapter, email, config)