Specify passing version of Sprockets

Not sure why it wasn't used in master, likely due to travis cache. The tests fail when Sprockets 3.0.0 is used but pass with 3.0.3, here's the bundle install diff https://www.diffchecker.com/36paaa47.

Author: schneems

gemfiles/sprockets_3_0.gemfile

@@ -1,6 +1,6 @@
 source 'https://rubygems.org'
 
-gem "sprockets", "~> 3.0.0"
+gem "sprockets", "~> 3.0.3"
 
 # Specify your gem's dependencies in sassc-rails.gemspec
 gemspec path: "../"