Use explicit patch level to set default ruby

References https://github.com/rails/rails-dev-box/issues/16

### Summary

Installing ruby with `--latest-binary` does not guarantee the latest patchlevel, and rvm is *extremely* specific about what it allows when you don't specify a patchlevel:

```
vagrant@rails-dev-box:~$ rvm list

rvm rubies

   ruby-1.9.3-p374 [ i686 ]
```
```
vagrant@rails-dev-box:~$ rvm use 1.9.3
ruby-1.9.3-p392 is not installed.
To install do: 'rvm install ruby-1.9.3-p392'
```

### Proposed Solution

When setting the default ruby, look at rvm's installed rubies to determine the exact value to pass to `rvm create alias default`