Update Cowboy to 2.x

[pablo-co]

Cowboy is now found on the plug_cowboy package.

[pablo-co]

Ebert has finished reviewing this Pull Request and has found:

• 1 possible new issue (including those that may have been commented here).

You can see more details about this review at https://ebertapp.io/github/pablo-co/bamboo_postmark/pulls/23.