Rakefile: Define Rubocop task

Rakefile

 require 'rubygems'
 require 'puppetlabs_spec_helper/rake_tasks'
 require 'puppet-lint/tasks/puppet-lint'
+require 'rubocop/rake_task'
 
 ENV['SPEC_OPTS'] = '--format documentation' unless ENV['SPEC_OPTS']
 
 PuppetSyntax.exclude_paths            = ['vendor/**/*.*']
 PuppetLint.configuration.ignore_paths = %w(spec/**/*.pp pkg/**/*.pp vendor/**/*.pp)
+RuboCop::RakeTask.new
 
 # Alternative configuration until https://github.com/rodjek/puppet-lint/pull/397 gets merged
 Rake::Task[:lint].clear
@@ -13,4 +15,4 @@ PuppetLint::RakeTask.new :lint do |config|
   config.ignore_paths = PuppetLint.configuration.ignore_paths
 end
 
-task :all => [:validate, :lint, :spec, :rubocop]
+task all: [:validate, :lint, :spec, :rubocop]