Add ActionMailer 6.0.0.rc2 build
Add ActionMailer 6.0.0.rc2 build

By building master, we are actually building 6.1.0 which is what master is right now but is not released.

fphilipe/premailer-rails,controlshift/premailer-rails,fphilipe/premailer-rails,fphilipe/premailer-rails,controlshift/premailer-rails,controlshift/premailer-rails