Update to sassc rails

[p8]

Continuing #287 by @cparker-ajla

I've replaced 'sass' with 'sassc' in the gemspec.

The "You are trying to install in deployment mode after changing your Gemfile" errors have been fixed by running: bundle exec appraisal install. The error occured because when Travis tries to runbundler install on the appraisals Gemfiles it would change the appraisals Gemfile.lock. Running: bundle exec appraisal install and commiting the lock files prevents this error.

Ruby 2.3.8 has been removed from the build matrix because it is end-of-life and fails with Rails 6.

Rails 6 requires some more work. See #282