Remove "ruby-" prefix from `.ruby-version` file

When running `bundle install` I get:

```
warning: ignoring extraneous `ruby-' prefix in version `ruby-2.1.0'
         (set by /Users/benbalter/projects/playbook/.ruby-version)
```

Running `rbenv local 2.1.0` (to set the rbenv version to `2.1.0`), it updates the `.rbenv-version` file contents to

`2.1.0`

Running `bundle install` now throws no warnings. :smile: