Add explicit dependency on sprockets.

Author: chriseppstein

sass-rails.gemspec

@@ -18,6 +18,7 @@ Gem::Specification.new do |s|
   s.add_runtime_dependency 'railties',   '~> 3.1.0'
   s.add_runtime_dependency 'actionpack', '~> 3.1.0'
   s.add_runtime_dependency 'tilt',       '~> 1.3.2'
+  s.add_runtime_dependency 'sprockets',  '~> 2.0.0'
 
   s.files         = `git ls-files`.split("\n")
   s.test_files    = `git ls-files -- {test,spec,features}/*`.split("\n")