Log Message:
Update rubygems to 1.3.1.

New features since 1.2.0:
* RubyGems doesn't print LOCAL/REMOTE titles for `gem query` and friends if
  stdout is not a TTY, except with --both.
* Added Gem.find_files, allows a gem to discover features provided by other
  gems.
* Added pre/post (un)install hooks for packagers of RubyGems.  (Not for gems
  themselves).
* RubyGems now installs gems into ~/.gem if GEM_HOME is not writable.  Use
  --no-user-install command-line switch to disable this behavior.
* Fetching specs for update now uses If-Modified-Since requests.
* RubyGems now updates the ri cache when the rdoc gem is installed and
  documentation is generated.