pom.xml: updated Spring Security to 3.1.6

@see https://jira.spring.io/secure/ReleaseNote.jspa?projectId=10040&version=13618 (3.1.4)
@see https://jira.spring.io/secure/ReleaseNote.jspa?projectId=10040&version=14135 (3.1.5)
@see https://jira.spring.io/secure/ReleaseNote.jspa?projectId=10040&version=14368 (3.1.6)

No functional changes.
Configure liquibase to use slf4j for logging.

@see http://blog.projectnibble.org/2013/12/29/fixing-liquibase-logging-in-spring/