Remove Sprockets hard dependency, allow Propshaft

lib/crono.rb

@@ -3,7 +3,6 @@ module Crono
 end
 
 require 'rails'
-require 'sprockets/railtie'
 require 'active_support/all'
 require 'crono/version'
 require 'crono/engine'

lib/crono/engine.rb

@@ -3,7 +3,11 @@ module Crono
     isolate_namespace Crono
 
     initializer 'crono.assets.precompile' do |app|
-      app.config.assets.precompile += %w( crono/application.css crono/materialize.min.css )
+      if app.config.respond_to?(:assets)
+        app.config.assets.precompile += %w( crono/application.css crono/materialize.min.css )
+      else
+        fail "Crono requires either Propshaft or Sprockets to be installed."
+      end
     end
 
     config.generators do |g|