pom.xml: fix Jackson dependency
pom.xml: fix Jackson dependency

2.6.0 is expected by WireMock.

Signed-off-by: Pierre-Alexandre Meyer <ff019a5748a52b5641624af88a54a2f0e46a9fb5@mouraf.org>

killbill/killbill-adyen-plugin,andrenpaes/killbill-adyen-plugin,BLangendorf/killbill-adyen-plugin