Downgrade sassc to version 2.1.0

Version 2.2.x seems to cause some issues with Ruby 2.6, as mentioned in
the comments of issue 146 in the sassc-ruby repository.

Due to these issues, our test suite was failing sometimes when running
on GitHub Actions.

Gemfile.lock

@@ -524,7 +524,7 @@ GEM
     sass-listen (4.0.0)
       rb-fsevent (~> 0.9, >= 0.9.4)
       rb-inotify (~> 0.9, >= 0.9.7)
-    sassc (2.2.1)
+    sassc (2.1.0)
       ffi (~> 1.9)
     sassc-rails (2.1.2)
       railties (>= 4.0.0)