Update google-protobuf requirement from ~> 4.27 to ~> 4.28

Author: ntkme

sass-embedded.gemspec

@@ -42,5 +42,5 @@ Gem::Specification.new do |spec|
                                  '>= 3.2'
                                end
 
-  spec.add_dependency 'google-protobuf', '~> 4.27'
+  spec.add_dependency 'google-protobuf', '~> 4.28'
 end