Log Message:
Update rubygems package to 1.8.15.

=== 1.8.15 / 2012-01-06

* 1 bug fix:

  * Don't eager load yaml, it creates a bad loop. Fixes #256

=== 1.8.14 / 2012-01-05

* 2 bug fixes:

  * Ignore old/bad cache data in Version
  * Make sure our YAML workarounds are loaded properly. Fixes #250.

=== 1.8.13 / 2011-12-21

* 1 bug fix:

  * Check loaded_specs properly when trying to satisfy a dep

* 2 minor enhancements:

  * Remove using #loaded_path? for performance
  * Remove Zlib workaround for Windows build.