Revert "Use plaform mri_19 for debugger gem"

This reverts commit 2233f1456f.

Gemfile

@@ -39,12 +39,7 @@ instance_eval File.read local_gemfile if File.exists? local_gemfile
 platforms :mri do
   group :test do
     gem 'ruby-prof', '~> 0.11.2'
-  end
-end
-
-platforms :mri_19 do
-  group :test do
-    gem 'debugger' unless ENV['TRAVIS']
+    gem 'debugger' if !ENV['TRAVIS'] && RUBY_VERSION < "2.0"
   end
 end