Reorganize Gemfile dependencies

Gemfile

@@ -1,10 +1,15 @@
 source 'https://rubygems.org'
 
+gem 'rake'
+
+group :debug do
+  gem 'pry'
+  gem 'pry-byebug', platforms: :mri
+end
+
 group :development do
   gem 'rubocop', github: 'bbatsov/rubocop', branch: 'master'
   gem 'rubocop-cask', '~> 0.2'
-  gem 'pry'
-  gem 'pry-byebug', platforms: :mri
 end
 
 group :release do
@@ -16,6 +21,5 @@ group :test do
   gem 'minitest', '5.4.1'
   gem 'minitest-reporters'
   gem 'mocha', '1.1.0', :require => false
-  gem 'rake'
   gem 'rspec', '~> 3.0.0'
 end