Slightly improve boot time

rubocop and lefthook gems are added to the Gemfile for convenience, but
are not needed on runtime. Not requiring them saves some memory
allocations and boot time.

Gemfile

@@ -291,12 +291,12 @@ group :development, :test do
   gem 'pry-stack_explorer', '~> 0.6.0'
 
   # ruby linting
-  gem 'rubocop'
-  gem 'rubocop-rails'
-  gem 'rubocop-rspec'
+  gem 'rubocop', require: false
+  gem 'rubocop-rails', require: false
+  gem 'rubocop-rspec', require: false
 
   # git hooks manager
-  gem 'lefthook'
+  gem 'lefthook', require: false
 
   # Brakeman scanner
   gem 'brakeman', '~> 5.2.0'

spec/lib/rubocop/cop/open_project/use_service_result_factory_methods_spec.rb

@@ -26,6 +26,7 @@
 # See COPYRIGHT and LICENSE files for more details.
 #++
 
+require 'rubocop'
 require 'rubocop/rspec/shared_contexts'
 require 'spec_helper'
 require 'rubocop/rspec/support'