Ruby version managers don't need the ruby- prefix

In fact, rbenv will warn that it's being ignored. This patch removes the
prefix as modern Ruby managers don't need it.

```
warning: ignoring extraneous 'ruby-' prefix in version 'ruby-2.1.1'
         (set by /Users/steven/code/themes_on_rails/.ruby-version)
```
Merge pull request #10 from stevenharman/fix-ruby-version-file

Ruby version managers don't need the ruby- prefix