Replaces sass-rails with sassc-rails due to gem deprecation.

[rlgreen91]

This change will replace the sass-rails gem with the sassc-rails gem. As noted here, the original Ruby implementation of Sass has been deprecated and EOL was in March. The sass-rails gem is supposed to get an upgrade to sassc, but it's unclear when that will happen, so the change to the gem was made. The only other gem we have that depends on sass-rails is rails_admin, but based on this it's unclear when or if that change will occur.

Edit: Actually, it should be noted that rails_admin has been updated to use sassc-rails as part of 2.0.0.rc - as such, we will wait for the official release of version 2 before upgrading that gem.

In your PR did you:

• Include a description of the changes?
• Mention the issue the PR addresses?
• Include screenshots of any changes to the UI?
• Isolate any changes to gems (meaning that any new, updated, or removed gems and resulting code changes should be in their own PR)?
• Add and/or update specs for your code?

[trystant]

LGTM