Bump to v3.1.0

[MatheusBueno782]

This release contains:

• Fix for using custom config with response: true by bumping bamboo version to ~> 1.6 (Bump bamboo 1.6 #150)
• Implement our custom test adapter ([Create a custom test adapter. #151])
• Fix CI random failure by attaching FakeGenSMTP Server process to Test supervision tree.(Attach FakeGenSMTP to test supervision tree #153)
• Add Content-ID header when needed(Add Content-ID header when needed #154)
• Base 64 encode the headers only when the content contains non-ASCII characters.([CU-9ux4p3] Base 64 encode the headers only when necessary #155)
• Handle :permanent_failure exception and re-raising it as a SMTPError.([CU-9ux5yt] Fix unhandled exception #156)
• After bumping the dependencies, the project requires(chore: update mix.exs dependencies #149):
  • credo ~> 1.4.1
  • bamboo ~> 1.6
  • excoveralls ~> 0.13.3
  • gen_smtp ~> 1.0.1

[kdisneur]

We should also bump the version in the README