Disable automatic loading of mail_room gem

This gem isn't needed inside GitLab, and excluding it avoids some
"already initialized constant" errors due to conflicting implementations
of `net/imap`. `bin/mail_room` already requires this gem.

Gemfile

@@ -439,7 +439,7 @@ end
 gem 'octokit', '~> 4.15'
 
 # https://gitlab.com/gitlab-org/gitlab/issues/207207
-gem 'gitlab-mail_room', '~> 0.0.10', require: 'mail_room'
+gem 'gitlab-mail_room', '~> 0.0.10', require: false
 
 gem 'email_reply_trimmer', '~> 0.1'
 gem 'html2text'