Remove ruby- prefix to stop rbenv from complaining. 

This has been verified as still being compatible with rvm.

Prior to this change, rbenv would give the following warning:

```bash
|2.1.1| natoma in ~/workspace/thrust
± ms+yt |master ✓| → pwd
    /Users/pivotal/workspace/thrust
    warning: ignoring extraneous `ruby-' prefix in version `ruby-2.1.1'
             (set by /Users/pivotal/workspace/thrust/.ruby-version)
```
As recommended in https://github.com/sstephenson/rbenv/issues/380, we've removed the prefix.