Only test that clean task on sprockets 2

sprockets 3 also take in consideration the age of the asset before
cleaning. By default it is one hour so we can't simulate it on the tests
anymore

Author: rafaelfranca

test/test_task.rb

@@ -106,6 +106,7 @@ def test_clean
     assert File.exist?("#{@dir}/#{digest_path}")
   end
 
+  if Sprockets::VERSION < "3"
   def test_clean_with_keep_specified
     assert !@environment_ran
     path     = @assets['foo.js'].pathname
@@ -139,4 +140,5 @@ def test_clean_with_keep_specified
   ensure
     FileUtils.rm(new_path) if new_path
   end
+  end
 end