DEV: upgrades sassc to 2.4.0 (#10447)

We were stuck on 2.0.1 but multiple reports on the original issue shows that the issue has been fixed since 2.3 and 2.4:

https://github.com/sass/sassc-ruby/issues/146#issuecomment-654522157
https://github.com/sass/sassc-ruby/issues/146#issuecomment-654522157

There's one report show it was not fixed in 2.3.0, but that's the only one, it's probably worth trying: https://github.com/sass/sassc-ruby/issues/146#issuecomment-639073736

Gemfile.lock

@@ -376,9 +376,8 @@ GEM
       crass (~> 1.0.2)
       nokogiri (>= 1.8.0)
       nokogumbo (~> 2.0)
-    sassc (2.0.1)
+    sassc (2.4.0)
       ffi (~> 1.9)
-      rake
     sassc-rails (2.1.2)
       railties (>= 4.0.0)
       sassc (>= 2.0)
@@ -544,7 +543,7 @@ DEPENDENCIES
   ruby-prof
   ruby-readability
   rubyzip
-  sassc (= 2.0.1)
+  sassc (= 2.4.0)
   sassc-rails
   seed-fu
   shoulda-matchers