Use .ruby-version file

I wanted to try out OctoPress and after cloning the repository I got a scary warning from rvm about trusting the `.rvmrc` file.  

I'm a Ruby n00b, so I'm just going to quote StackOverflow:

> If your .rvmrc file contains custom shell code, continue using .rvmrc as it allows you to include any shell code.

> If your only aim is to switch Ruby versions, then use .ruby-version which is supported by other Ruby version switchers such as rbenv or chruby. This file also does not require trusting as it is just the name of a Ruby version and will not be executed in any way.

http://stackoverflow.com/questions/15708916/use-rvmrc-or-ruby-version-file-to-set-a-project-gemset-with-rvm/15709012#15709012