Update Rakefile to use new Rubocop class name

Author: tas50

Rakefile

@@ -21,7 +21,7 @@ YARD::Rake::YardocTask.new do |t|
   t.options = %w(--markup-provider=redcarpet --markup=markdown --main=README.md --files CHANGELOG.md)
 end
 
-Rubocop::RakeTask.new
+RuboCop::RakeTask.new
 
 RSpec::Core::RakeTask.new(:spec) do |r|
   r.pattern = FileList['**/**/*_spec.rb']