Not compatible with Erlang/OTP 24

[marcandre]

Erlang/OTP 24 dropped :crypto.hmac.

=> ex_twilio
Compiling 74 files (.ex)
warning: :crypto.hmac/3 is undefined or private, use crypto:mac/4 instead
  lib/ex_twilio/request_validator.ex:35: ExTwilio.RequestValidator.compute_hmac/2

One must use :crypto.mac/4 which is OTP 22+

FYI, the package plug_crypto was updated to tackle the same thing in elixir-plug/plug_crypto@cf901bd and elixir-plug/plug_crypto@8c1f927

Let me know if I need to write a PR for this.

[seantanly]

This is addressed in PR #154
Will be nice if that's merged and we get a new release.

[marcandre]

Oh, perfect. Closing in favor of #147 / #154