You can specify the latest version of ruby by looking at https://www.ruby-lang.org/en/downloads/
Fetch the latest version:
curl -sSL https://get.rvm.io | bash -s stable --ruby
Install it:
rvm install 2.2
Use it as default:
rvm use 2.2 --default
Or run the latest command from ruby:
rvm install ruby --latest
rvm use 2.2 --default