ddl: drop NOT NULL constraint on z_snd_reference_id

Signed-off-by: Pierre-Alexandre Meyer <pierre@ning.com>
pom.xml: update nexus-staging-maven-plugin to 1.4.3

See https://github.com/sonatype/announcements/blob/master/ossrh/2013-04-16-ossrh-upgrade.md

Signed-off-by: Pierre-Alexandre Meyer <pierre@ning.com>