Install bootstrapped ruby into buildpack-specific namespace

[robdimsdale]

This buildpack bootstraps ruby into /tmp/ruby, and there are situations where multiple buildpacks attempt to bootstrap ruby into the same location. This may be causing non-deterministic issues, so to be safe we should bootstrap into /tmp/<buildpack-name>/ruby.

See example of bootstrapping ruby into buildpack-specific namespace: cloudfoundry/php-buildpack#855