repl: keep the built-in modules non-enumerable

Make sure that the built-in modules in the repl stay non-enumerable.
Previously, they would pop up as enumerable properties of the global
object after having been accessed for the first time.

PR-URL: https://github.com/nodejs/node/pull/6207


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>