pom.xml: update logback to 1.1.7 and slf4j to 1.7.21
pom.xml: update logback to 1.1.7 and slf4j to 1.7.21

See https://github.com/killbill/killbill/issues/288.

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

killbill/killbill-oss-parent