Add .ruby-version file

A `.ruby-version` file is a helpful way to know what version of Ruby to use in a given project. This file gets picked up by tools like RVM and rbenv.

I recently started trying to contribute to this project, but ran into some issues when trying to install the `debugger` gem. Eventually I realized that you must have been using a different version of Ruby than I was, so you weren't experiencing these same errors.

I have added a `.ruby-version` file with a known working Ruby version to help contributors know what to expect.