models: more fixes for Kaui

Add support for invoice item adjustments and CBA deletion.

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
Merge branch 'master' of github.com:killbill/killbill-client-ruby
models: more fixes for Kaui

Add support for bundle tags and custom fields.

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>