Update rbenv-version to use 1.9.3-p374 / Rename to ruby-version

Current rbenv-version file contains 1.9.3-p194, which is quite behind the latest version and  has several security vulnerabilities. ([ref](http://www.ruby-lang.org/en/news/2012/))

Also renamed .rbenv-version to .ruby-version, as recommended by rbenv ([ref](https://github.com/sstephenson/rbenv/)).