Add to Rubygems

[vernondegoede]

It would be nice if all spree_mail_settings would be added to Rubygems.org. Right now, installing this Gem via Github will result in dependency conflicts:

Bundler could not find compatible versions for gem "spree_backend":
  In snapshot (Gemfile.lock):
    spree_backend (= 3.0.9)

  In Gemfile:
    spree (~> 3.0.9) was resolved to 3.0.9, which depends on
      spree_backend (= 3.0.9)

    spree_mail_settings was resolved to 3.2.0.alpha, which depends on
      spree_backend (~> 3.2.0.alpha)

    spree_mail_settings was resolved to 3.2.0.alpha, which depends on
      spree_backend (~> 3.2.0.alpha)

Running `bundle update` will rebuild your snapshot from scratch, using only
the gems in your Gemfile, which may resolve the conflict.

[mvastola]

Could this please be done?