Require sprockets-rails >= 2

Fixes rails@4d157ea

Without any specification about the version of sprockets-rails, running a
`bundle install` on a brand new app might result in sprockets 0.0.1 being
installed.

However, the minimum requirement is sprockets-rails 2 (see https://github.com/rails/rails/pull/17752/files)

Author: claudiob

Gemfile.lock

@@ -139,7 +139,7 @@ PATH
       activesupport (= 5.0.0.alpha)
       bundler (>= 1.3.0, < 2.0)
       railties (= 5.0.0.alpha)
-      sprockets-rails
+      sprockets-rails (>= 2.0.0)
     railties (5.0.0.alpha)
       actionpack (= 5.0.0.alpha)
       activesupport (= 5.0.0.alpha)

rails.gemspec

@@ -28,5 +28,5 @@ Gem::Specification.new do |s|
   s.add_dependency 'railties',      version
 
   s.add_dependency 'bundler',         '>= 1.3.0', '< 2.0'
-  s.add_dependency 'sprockets-rails'
+  s.add_dependency 'sprockets-rails', '>= 2.0.0'
 end