Bumping passenger version doesn't recalculate attributes

[jaylevitt]

When I first installed rvm_passenger, it chose passenger 3.0.8. I changed my recipe to explicitly set 3.0.14, but /etc/apache2/mods-enabled/passenger.conf and passenger.load are still pointing at 3.0.8.

I think that's because the not_if guards around root_path and module_path cause them to be set only on the first run. I'd contribute a fix but I don't know the idiomatic chef way to calculate these values only when needed (I presume you don't want to set them every time, as that'd be a useless round trip to the server?)