discordrb depends on rest-client here:

This library hasn't been updated in 6 years (commits link):

rest-client also depends on http-accept < 2.0.0, so using discordrb as-is means that an outdated http-accept library is also used (permalink):

  s.add_dependency('http-accept', '>= 1.7.0', '< 2.0')

Instead of rest-client, I recommend using Faraday, or possibly just using ordinary Net:HTTP 👍