Don't try to load byebug in non-MRI envs.

Gemfile

@@ -3,13 +3,13 @@ source "https://rubygems.org"
 group :development, :test do
   gem 'rake'
   gem 'mongoid', '2.6.0'
-  gem 'bson_ext', :platforms => :ruby
+  gem 'bson_ext', platforms: :ruby
   gem 'geoip'
   gem 'rubyzip'
   gem 'rails'
   gem 'test-unit' # needed for Ruby >=2.2.0
 
-  gem 'byebug'
+  gem 'byebug', platforms: :mri
 
   platforms :jruby do
     gem 'jruby-openssl'