Update ruby-rspec-core to 2.9.0.

Update ruby-rspec-core to 2.9.0.

### 2.9.0 / 2012-03-17
[full changelog](http://github.com/rspec/rspec-core/compare/v2.8.0...v2.9.0)

Enhancements

* Support for "X minutes X seconds" spec run duration in formatter. (uzzz)
* Strip whitespace from group and example names in doc formatter.
* Removed spork-0.9 shim. If you're using spork-0.8.x, you'll need to upgrade
  to 0.9.0.

Bug fixes

* Restore `--full_backtrace` option
* Ensure that values passed to `config.filter_run` are respected when running
  over DRb (using spork).
* Ensure shared example groups are reset after a run (as example groups are).
* Remove `rescue false` from calls to filters represented as Procs
* Ensure described_class gets the closest constant (pyromaniac)
* In "autorun", don't run the specs in the at_exit hook if there was an
  exception (most likely due to a SyntaxError). (sunaku)
* Don't extend groups with modules already used to extend ancestor groups.
* `its` correctly memoizes nil or false values (Yamada Masaki)