update to posting guide: use 'sessionInfo()' instead of 'version'

I think using sessionInfo() instead of version is a good idea.

[Note that sessionInfo() currently does not report all the
information that 'version' does (it omits at least "Status" and "svn
rev").  R-core members are aware of this -- whether or not they
change this is up to them.]

Another thing not currently reported by sessionInfo() is a list of
loaded name spaces.  As more packages use name spaces and importing
dependencies instead of attaching them, this will become useful debug
info.

Loaded name spaces can be listed using loadedNamespaces().

+ seth