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

`Rbenv` complains after each command

```console
warning: ignoring extraneous `ruby-' prefix in version `ruby-2.1.0'
         (set by /Users/seanpdoyle/src/formulaic/.ruby-version)
```
Merge pull request #22 from seanpdoyle/sd-fix-ruby-version-for-rbenv

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