pom.xml: updates for release 0.19.15
pom.xml: updates for release 0.19.15

Fix issue in usage billing (DETAIL mode)
Catalog endpoints enhancements
Merge fix from 0.18.x to allow to change to a plan defined in a subsequent catalog version

Signed-off-by: Kill Bill core team <1a73af9e7ae00182733b2292511b814be66f065f@killbill.io>

sbrossie/killbill,sbrossie/killbill,killbill/killbill,sbrossie/killbill,sbrossie/killbill,killbill/killbill,killbill/killbill,killbill/killbill,sbrossie/killbill,killbill/killbill