Don't shell out to RVM

Without this patch, I can't even run `rake -T`:

```
~/tmp/builder master 2.1.0-github1 λ rake -T
rake aborted!
No such file or directory - rvm
/Users/charlie/tmp/builder/rakelib/tags.rake:15:in ``'
/Users/charlie/tmp/builder/rakelib/tags.rake:15:in `<module:Tags>'
/Users/charlie/tmp/builder/rakelib/tags.rake:3:in `<top (required)>'
(See full trace by running task with --trace)
```