Add Ruby version so to declare streamline version.

As there are some issues with development tools (like flog disabled in devtools
for MRI 2.0.0), I'm adding here a `.ruby-version` file so we can align on what
the streamline dev Ruby is and what is to be considered _compatible_.

This file isn't here to enforce the use of any automatic switching tools like
rbenv or chruby (which I personally encourage), but to set a sync point so to
know how to bother with possible compatibility issues.

So, consider this PR as a starting point for a discussion, and feel free to set another version as streamline.