Move requirement rubocop into proper group.

Gemfile

@@ -44,13 +44,13 @@ group :development, :test do
   # Call 'byebug' anywhere in the code to stop execution and get a debugger console
   gem 'byebug', platforms: %i[mri mingw x64_mingw]
   gem 'mina', '1.2.3'
+
+  # Use Rubocop to lint files
+  gem "rubocop", "~> 1.16.0"
 end
 
 group :development do
 end
 
 # Windows does not include zoneinfo files, so bundle the tzinfo-data gem
 gem 'tzinfo-data', platforms: %i[mingw mswin x64_mingw jruby]
-
-# Use Rubocop to lint files during developing but it is not required in production
-gem "rubocop", "~> 1.16.0", require: false