rspec-core-2.3.1 is released

### rspec-core-2.3.1 / 2010-12-16

full changelog: http://github.com/rspec/rspec-core/compare/v2.3.0...v2.3.1

* Bug fixes
  * send debugger warning message to $stdout if RSpec.configuration.error_stream
    has not been defined yet.
  * HTML Formatter _finally_ properly displays nested groups (Jarmo Pertman)
  * eliminate some warnings when running RSpec's own suite (Jarmo Pertman)