Lock byebug to version 9.0.6 with Rails 4.1.

gemfiles/Gemfile.rails4.1

@@ -9,7 +9,7 @@ group :development, :test do
   gem 'rails', '~> 4.1.13'
   gem 'test-unit' # needed for Ruby >=2.2.0
 
-  gem 'byebug', platforms: :mri
+  gem 'byebug', '9.0.6', platforms: :mri
 
   platforms :jruby do
     gem 'jruby-openssl'