Use latest sprockets 3.7.x release

This was many minor versions behind. We want to ensure our CI uses the current minor version.

Author: javierjulio

gemfiles/sprockets_3_0.gemfile

@@ -1,6 +1,6 @@
 source 'https://rubygems.org'
 
-gem "sprockets", "~> 3.0.3"
+gem "sprockets", "~> 3.7"
 
 # Specify your gem's dependencies in sassc-rails.gemspec
 gemspec path: "../"