Remove ActionMailer 4 version from build matrix
Remove ActionMailer 4 version from build matrix

The AM4/Ruby2.7 build failed because of bundler version: https://travis-ci.org/github/fphilipe/premailer-rails/jobs/663565100#L241-L252 and preference was to drop the build to resolve issue.

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