Bump premailer-rails gem to 1.9.7 and its dependencies to prevent network retrieval of assets

Since asset compilation was disabled in GitLab 9.3, the premailer-rails gem
would fall back to fetching assets via HTTP(s) since the Sidekiq working
directory was not the Rails root. In some instances, SSL verification would
fail, preventing e-mails from being sent.

premailer-rails 1.9.7 has a fix
(https://github.com/fphilipe/premailer-rails/pull/195) that uses an absolute
directory to load assets from the filesystem so that the assets can be fetched
locally instead of over the network.

Closes #34231