Update 2.0.0 stdlib to p195, plus other fixes.

* Add patchlevel to 2.0.0 version string.
* Set up RUBY_REVISION appropriate for 2.0.0.
* Fix setting build-time Ruby version to 2.0 (cc @rkh, @mpapis)
* Fix setting default Ruby version to 2.0 (cc @khaase)