Ruby Installation directory set-able via environment variable

I added the `RY_RUBIES` environment variable that defaults to `$RY_PREFIX/lib/ry/rubies`.

Note that I applied the version bump to `0.4.0` as I am assuming you are using semantic versioning, in which case this update should be a backward-compatible feature change.

I wasn't quite sure what magic is going on in `ry::fullpath`, so please advise if it I inadvertantly broke it.