fix(rb-readline): add 'rb-readline' as dev dependency to fix 'byebug'…

… installation

- when installing (on Mac OS Sierra for instance), `byebug` generated an error since it required `readline` which wasn't installed
- see `byebug` related issue
@link deivid-rodriguez/byebug#289

minimal.rb

@@ -26,6 +26,7 @@
   gem 'better_errors'
   gem 'binding_of_caller'
   gem 'listen', '~> 3.0.5'
+  gem 'rb-readline'
   gem 'pry-byebug'
   gem 'pry-rails'
   gem 'spring'