NoMethodError: undefined method `ignore' for Warning:Module

I am unable to find any explanation for this error but when I am running: rake test or bundle exec rake test I am getting:

NoMethodError: undefined method `ignore' for Warning:Module
/Users/przemyslaw/projects/writefully/config/initializers/warning.rb:4:in `block in <top (required)>'
/Users/przemyslaw/projects/writefully/config/initializers/warning.rb:3:in `each'
/Users/przemyslaw/projects/writefully/config/initializers/warning.rb:3:in `<top (required)>'
/Users/przemyslaw/projects/writefully/config/environment.rb:5:in `<top (required)>'
/Users/przemyslaw/projects/writefully/test/test_helper.rb:2:in `require'
/Users/przemyslaw/projects/writefully/test/test_helper.rb:2:in `<top (required)>'
/Users/przemyslaw/projects/writefully/test/controllers/comments_controller_test.rb:1:in `require'
/Users/przemyslaw/projects/writefully/test/controllers/comments_controller_test.rb:1:in `<top (required)>'
Tasks: TOP => test:run
(See full trace by running task with --trace)

I am following a tutorial on codemy.net and there is nothing complicated or special about this project. I have only generated a new rails app, a couple of models and controllers and now when I am trying to run the test it gives me this error. There is no single test within my test files at the moment so this is nothing to do with testing. It looks like there is some issue with the 'ignore' method within the warning file:

config/initializers/warning.rb

# Rails will load this logic before booting up Rails server
unless Rails.env.development?
  Gem.path.each do |path|
    Warning.ignore(//, path)
  end
end

Any help or idea would be appreciated. I am using rails 4.2.10 and ruby 2.5.8.