Support gem execution with rbenv ruby.

This change will enable the gem module to use rbenv's installed ruby versions, just like rvm is supported already today. This feature was approved in issue 7681.

Please note: I have only tested this by overriding modules locally in a _modules folder. It works as expected. But I have not set up the full development environment, or run the tests, for this change. Perhaps, since the change is very contained that could still be acceptable?