Switch to byebug for MRI 2.x debugging. Debugger is not supported for 2.1.2...

Switch to byebug for MRI 2.x debugging.  Debugger is not supported for 2.1.2 and up, and was never fully 2.x compatible.

Gemfile

@@ -8,9 +8,8 @@ group :development, :test do
   gem 'rubyzip'
   gem 'rails'
 
-  platforms :mri do
-    gem 'debugger'
-  end
+  gem 'debugger', :platforms => [:mri_19]
+  gem 'byebug', :platforms => [:mri_20, :mri_21]
 
   platforms :jruby do
     gem 'jruby-openssl'