pom.xml: updated Spring-Data-JPA to 1.6.4

This also updated Spring-Data-Commons to 1.8.4.

@see http://docs.spring.io/spring-data/jpa/docs/1.6.4.RELEASE/changelog.txt
@see http://docs.spring.io/spring-data/commons/docs/1.8.4.RELEASE/changelog.txt

No functional changes.