Don't shell out to RVM

[haileys]

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)

[davispuh]

+1
not everyone have rvm

[kbrock]

this has been resolved on https://github.com/tenderlove/builder