use rack from master branch

it's needed until new rack release with Rack::BodyProxy fixes
b06ef82876

Gemfile

@@ -2,6 +2,8 @@ source 'https://rubygems.org'
 
 gemspec
 
+gem 'rack', :git => 'git://github.com/rack/rack.git'
+
 if ENV['AREL']
   gem 'arel', :path => ENV['AREL']
 else

railties/lib/rails/generators/rails/app/templates/Gemfile

@@ -2,6 +2,8 @@ source 'https://rubygems.org'
 
 <%= rails_gemfile_entry -%>
 
+gem 'rack', :git => 'https://github.com/rack/rack.git'
+
 <%= database_gemfile_entry -%>
 
 <%= "gem 'jruby-openssl'\n" if defined?(JRUBY_VERSION) -%>