enabling ensure_gem with system-level Rubies

It's not clear this is the best way to handle it, but it doesn't seem terrible. It came up in using Terraform to set up our dev/test environment: I couldn't get rbenv working smoothly in an automated way, and then realized that we're using Vagrant VMs that are both disposable and specific to the system in question, so we should just use the Ruby we use in production (which we install from apt).