Respect RUBIES_DIR environment variable

In `scripts/setup.sh` it is possible to override the location where Rubies will be installed with `RUBIES_DIR`.

````bash
# script/setup.sh:18
[[ -z "$RUBIES_DIR"  ]] && export RUBIES_DIR="/opt/rubies"
````

However, in `chruby.sh` only hard coded locations are checked. This patch adds a similar check to `chruby.sh`.

I am aware that introducing new environment variables is discouraged, however since this one is already in use, but not used consistently, I would like to propose this change.