Only load debugger in MRI.

Gemfile

@@ -2,13 +2,16 @@ source "https://rubygems.org"
 
 group :development, :test do
   gem 'rake'
-  gem 'debugger'
   gem 'mongoid', '2.6.0'
   gem 'bson_ext', :platforms => :ruby
   gem 'geoip'
 
   gem 'rails'
 
+  platforms :mri do
+    gem 'debugger'
+  end
+
   platforms :jruby do
     gem 'jruby-openssl'
   end