RVM Support

The `rvm` branch and this pull request will eventually make Prax correctly handle RVM.

The first commit removes any specific Ruby from `$PATH` which allows Prax to run Racker from a clean shell which should let RVM do it's job of choosing the correct ruby and gemset for the project.

This looks like to be enough with a simple Rack application (setting .ruby-version and restarting the instance actually selects the correct Ruby version):

```ruby
run(lambda do |env|
  [200, {}, [RUBY_VERSION, "\n"]]
end)
```
if you are about to merge it better move the `set -e` after sourcing RVM