Relax Rack dependency

Rack is very carefully released, we should be able to upgrade minor
versions without much effort. We are a bunch of Rails core who are also
Rack core members so there won't be any issue with that. And in case
there's something wrong, we should fix on both sides.
Even though, doesn't seem like we will have a 1.7 version, this will be
useful as an example for when we go with Rack 2.0. We should ~> 2.0.

actionpack/actionpack.gemspec

@@ -21,7 +21,7 @@ Gem::Specification.new do |s|
 
   s.add_dependency 'activesupport', version
 
-  s.add_dependency 'rack',      '~> 1.6.0'
+  s.add_dependency 'rack',      '~> 1.6'
   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'