removes .ruby-version file

This removes the `.ruby-version` file.

At the moment this forces people using rbenv to install ruby 1.9.3-p125 (which is a bit old) or live with a file change in git's index and remember to never commit it.

This file should normally be added to `.gitignore` to allow users to select their preferred version of ruby. I can add another commit here.
In my own setup I've added this file to the global gitignore, but it's probably best to add it in the repository to prevent people from submitting code that brings it back in.

Would love your opinion!