Update doc/source/r_interface.rst
rpy2-2.3.0 is becoming the current release.
Changes to the API are relatively few. 

On that topic, I may be willing to port the code to rpy2.3 since I use the conversions a lot. Where are the major API changes?
 API changes should be listed [here](http://rpy.sourceforge.net/rpy2/doc-2.3/html/changes.html#new-features)
