rename .rbenv-version file

https://github.com/sstephenson/rbenv#version-history

**0.4.0** (January 4, 2013)

* rbenv now prefers `.ruby-version` files to `.rbenv-version` files for specifying local application-specific versions. The `.ruby-version` file has the same format as `.rbenv-version` but is [compatible with other Ruby version managers](https://gist.github.com/1912050).