Restore MBCHARS env var to CI

[lewispb]

When we used Travis CI we had the MBCHARS environment variable as part of the matrix options.

This environment variable controls the Mail::Multibyte.proxy_class that mail uses.

https://github.com/mikel/mail/blob/6e5db59a349a2af9146d16d8f66cbd3b81cd905a/spec/environment.rb#L17-L24

This PR restores the ability to test on CI all Ruby versions against either Mail::Multibyte.proxy_class option.

[jeremy]

So, cool story… looks like we aren't using the Multibyte::Chars backend at all anymore. They stuck around for backward compatibility since they were publicly exposed, but usage was removed in #1094 and #1095.

[jeremy]

Closing this out since we aren't exercising the multibyte chars proxies within the lib anymore. We do have some specs that check that other APIs behave when passed Multibyte::Chars instances, but this is a niche scenario. Slating this subsystem for removal is sensible at this point.