Remove dependence on ruby gems for sass

The plugin that is used for SASS compilation to CSS is no longer supported and the Ruby Gem can no longer be found on many environments. Several modules have started using node sass to do this process instead.

See:

• https://github.com/openmrs/openmrs-module-registrationapp/commit/dc409a7325460421e2ec8d53e954b6d2dca6626a
• https://github.com/PIH/openmrs-module-pihcore/pull/266
• https://github.com/PIH/openmrs-module-labtrackingapp/pull/24
• https://github.com/openmrs/openmrs-module-appointmentschedulingui/pull/33