Fix variables

1. Fix `root_path` to use `version`

Since `node['passenger']['root_path']` depends on `node['passenger']['version']`, the default value should be redefined in recipe. Without it, `root_path` will use different version from `version`. I'm not sure this is the best way - and there is duplicated code :( But it fixes the problem.

2. Fix passenger so path

Passenger 4.0.8 creates .so file in buildout, not ext directory. see https://github.com/FooBarWidget/passenger/commit/dcaf0ea5310e87c935dd39abd6502e00ba8b5ddd

I'm not sure from which version the output directory have been changed. There is no way to know the output directory :( passenger-config does not have option for it.