pom.xml(maven-{surefire,failsafe}-plugin): updated to 2.15

@see http://www.mail-archive.com/announce@maven.apache.org/msg00531.html
@see http://www.mail-archive.com/announce@maven.apache.org/msg00537.html
@see http://www.mail-archive.com/announce@maven.apache.org/msg00543.html

No functional changes.