Ruby and gems versions in the Gemfile

The Ruby on Rails Tutorial by Michael Hartl uses a Gemfile without specifying a Ruby version: the ruby keyword is missing in all the applications.

When I deploy to Heroku I receive the following warning:

remote: ###### WARNING:
remote:        You have not declared a Ruby version in your Gemfile.
remote:        To set your Ruby version add this line to your Gemfile:
remote:        ruby '2.2.4'
remote:        # See https://devcenter.heroku.com/articles/ruby-versions for more information.

The tutorial says that “* the
costs associated with including such an explicit Ruby version number outweigh the (negligible) benefits, so you should ignore this warning for now. The main issue is that keeping your sample app and system in sync with the latest Ruby version can be a huge inconvenience*”.

I am wondering what is better (if I can choose), between using the same Ruby version at Heroku and specifying any Ruby version it suits me. Since I am using Ruby 2.2.1p85 for my application, adding ruby "2.2.1", :patchlevel => "85" would work and make Heroku adjust to this version or rather I am supposed to adjust to Heroku and add as suggested ruby '2.2.4'? Can I leave out the Ruby version with no worries?

I am using rvm and a specific gemset with a specific Ruby version, for no other reason than trying to use an environment as much close to the tutorial as possible.

How can I decide what Ruby version and what version of the gems in the Gemfile are suitable for my application?

I prefer keeping ruby version in Gemfile even when I do not use Heroku