2.5.1 (unstable):

* Use (and require) C++11
  (Kjell Ahlstedt) 
* Using C++11 lambda functions to create sigc::slots:
  Avoid the need for SIGC_FUNCTORS_DEDUCE_RESULT_TYPE_WITH_DECLTYPE.
  (Kjell Ahlstedt)
* Remove the already-deprecated sigc::lambda (and sigc::group) API.
  This is an API change but should not be an ABI change, so it should
  only affect you when rebuilding applications, if you use this API.
  (Murray Cumming, Kjell Ahlstedt)