Use released rack

`rack.version` was bumped in https://github.com/rack/rack/commit/28e77c710ac2ddb35c436d78d72e28f28f964446

actionpack/actionpack.gemspec

@@ -21,7 +21,7 @@
 
   s.add_dependency 'activesupport', version
 
-  s.add_dependency 'rack',      '~> 1.6.0.beta2'
+  s.add_dependency 'rack',      '~> 1.6.0'
   s.add_dependency 'rack-test', '~> 0.6.2'
   s.add_dependency 'rails-html-sanitizer', '~> 1.0', '>= 1.0.1'
   s.add_dependency 'rails-dom-testing', '~> 1.0', '>= 1.0.5'

actionpack/test/dispatch/test_request_test.rb

@@ -18,7 +18,7 @@ class TestRequestTest < ActiveSupport::TestCase
     assert_equal "0.0.0.0", env.delete("REMOTE_ADDR")
     assert_equal "Rails Testing", env.delete("HTTP_USER_AGENT")
 
-    assert_equal [1, 2], env.delete("rack.version")
+    assert_equal [1, 3], env.delete("rack.version")
     assert_equal "", env.delete("rack.input").string
     assert_kind_of StringIO, env.delete("rack.errors")
     assert_equal true, env.delete("rack.multithread")