mention  .ruby-version for rbenv comment 

according to `rbenv` Readme https://github.com/sstephenson/rbenv :

Previous versions of rbenv stored local version specifications in a file named `.rbenv-version`. For backwards compatibility, rbenv will read a local version specified in an `.rbenv-version` file, but a `.ruby-version` file in the same directory will take precedence.
Merge pull request #249 from equivalent/patch-1

mention  .ruby-version for rbenv comment