Log Message:
Update to 2.3.1

Upstream changes:
# 2014-09-21 - Version 2.3.1
- Fixed message "Warning in formatC(Signif) : class of 'x' was discarded"
- Fixed problem looking up objects in Sweave. R Code inside
  Sweave files is now evaluated in .GlobalEnv, again, as
  before version 2.3.0.

# 2014-09-06 - Version 2.3
- API BREAK: Make package conform to new CRAN policies.
- .HTML.file is no longer stored in the global environment:
  use HTMLSetFile() and HTMLGetFile() instead of modifying it directly.
- .HTMLenv is no longer stored in the global environment:
  it should never have been accessed directly in the first place.
- fix() is no longer replaced during HTMLStart()/HTMLStop() sessions.
- Remove obsolete handling of 'date' class from package survival.
- Remove obsolete support for old format of objects from the rpart package