Support rake 11.x

Created by: tjgrathwell

The method Rake.application.last_comment has been removed in favor of Rake.application.last_description

Removal commit from rake: https://github.com/ruby/rake/commit/e76242ce7ef94568399a50b69bda4b723dab7c75

last_comment has been deprecated in favor of last_description since 2007 (rake 0.7.4): https://github.com/ruby/rake/commit/d1f89d7c62365633731850c8ec1ba76808d21cf8