Bump up version number to 3.1.1

3.1.0 ->
3.1.1

% make update-version NEW_VERSION=3.1.1
deb rpm: add 3.1.1 release entry

% make update-latest-release OLD_RELEASE=3.1.0 OLD_RELEASE_DATE=2013-11-29 NEW_RELEASE_DATE=2013-12-29
doc: update release version