Added support for .ruby-version to ensure congruent dev environment

It's important that all development happen under the same Ruby version (we have Travis to help with other versions if we so choose) so that all changes are compatible with the latest stable MRI.

It would be superfluous and undesirable to clutter up the repo with `.rbenv-version`, `.rvmrc` and other such files, but each of these Ruby Version Managers have made an effort to align with one central version file: `.ruby-version`.

This PR includes this `.ruby-version` file, which contains the latest MRI, `ruby-1.9.3-p362`.