How to detect what causes a deprecation warning in Rake
Try updating gem "rspec-rails", "~> 3.4.4", that resolved the deprecation warnings for me.
I find it easier modifying the the line from warn to raise in rake-11.1.0/lib/rake/task_manager.rb:10.
For me the offending gems are rspec (fixed in 3.4.4) and rubocop (fixed in 0.38.0).
You can recursively grep your gems, with:
grep -r last_comment /path/to/gems