Pass --linter=rubocop to bundle gem

It seems very unlikely that someone would want to generate a RuboCop
extension, but use a linter other than RuboCop, so let's save the user
the time.

lib/rubocop/extension/generator/generator.rb

@@ -7,7 +7,7 @@ def initialize(name)
         end
 
         def generate
-          system('bundle', 'gem', '--test=rspec',  name, exception: true)
+          system('bundle', 'gem', '--linter=rubocop', '--test=rspec',  name, exception: true)
 
           put "lib/#{name}.rb", <<~RUBY
             # frozen_string_literal: true