GRN2-45, GRN2-197: Upgrade Rails to 5.2.3 and removed sass-rails gem (#636)

* Removed deprecate sass-rails and replace with sassc-rails

* Upgraded to Rails 5.2.3

* Fixed travis

* Added back Spring code

* Fixed travis again

app/controllers/themes_controller.rb

@@ -29,7 +29,7 @@ class ThemesController < ApplicationController
     @file_contents = File.read(file_name)
 
     # Include the variables and covert scss file to css
-    @compiled = Sass::Engine.new("$primary-color:#{color};" \
+    @compiled = SassC::Engine.new("$primary-color:#{color};" \
                                  "$primary-color-lighten:#{lighten_color};" \
                                  "$primary-color-darken:#{darken_color};" +
                                  @file_contents, syntax: :scss).render