pom.xml: updates for release 0.51.7
pom.xml: updates for release 0.51.7

Add BillingPeriod.SIXTY_DAYS and BillingPeriod.NINETY_DAYS

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

killbill/killbill-api,killbill/killbill-api