Do not allow rack pre releases

Gemfile.lock

@@ -44,7 +44,7 @@ PATH
     actionpack (5.1.0.alpha)
       actionview (= 5.1.0.alpha)
       activesupport (= 5.1.0.alpha)
-      rack (~> 2.x)
+      rack (~> 2.0)
       rack-test (~> 0.6.3)
       rails-dom-testing (~> 2.0)
       rails-html-sanitizer (~> 1.0, >= 1.0.2)
@@ -221,7 +221,7 @@ GEM
     pg (0.18.4-x64-mingw32)
     pg (0.18.4-x86-mingw32)
     pkg-config (1.1.7)
-    psych (2.0.17)
+    psych (2.1.0)
     puma (3.4.0)
     qu (0.2.0)
       multi_json
@@ -231,8 +231,7 @@ GEM
       simple_uuid
     que (0.11.4)
     racc (1.4.14)
-    rack (2.0.0.rc1)
-      json
+    rack (2.0.1)
     rack-cache (1.6.1)
       rack (>= 0.4)
     rack-test (0.6.3)
@@ -292,7 +291,7 @@ GEM
       concurrent-ruby (~> 1.0)
       rack (> 1, < 3)
     sprockets-export (0.9.1)
-    sprockets-rails (3.0.4)
+    sprockets-rails (3.1.1)
       actionpack (>= 4.0)
       activesupport (>= 4.0)
       sprockets (>= 3.0.0)

actionpack/actionpack.gemspec

@@ -21,7 +21,7 @@ Gem::Specification.new do |s|
 
   s.add_dependency 'activesupport', version
 
-  s.add_dependency 'rack',      '~> 2.x'
+  s.add_dependency 'rack',      '~> 2.0'
   s.add_dependency 'rack-test', '~> 0.6.3'
   s.add_dependency 'rails-html-sanitizer', '~> 1.0', '>= 1.0.2'
   s.add_dependency 'rails-dom-testing', '~> 2.0'