(CFACT-122, CFACT-127) Fix ruby detection under rbenv and Fedora.

Fixing ruby detection under rbenv by executing "rbenv which ruby".  This
will allow us to use the ruby currently specified in the local or global
rbenv settings.

Fixing ruby detection under Fedora by also searching under "lib64".
FYI, this was changed otherwise we'd have a circular reference from api::instance -> execution::execute -> api::instance...