killbill_helper: use ruby-1.8 compatible syntax

Signed-off-by: Pierre-Alexandre Meyer <pierre@ning.com>
gemspec: update killbill-client to 0.1.1

Signed-off-by: Pierre-Alexandre Meyer <pierre@ning.com>
version: update to 0.2.3

Signed-off-by: Pierre-Alexandre Meyer <pierre@ning.com>