core: Upgrade maven-checkstyle-plugin to 2.9.1

Upgraded maven-checkstyle-plugin to 2.9.1 in order to facilitate a
future move to Java 7.
This version of maven-checkstyle-plugin requires a higher version of
checkstyle, so it was upgraded too, to 5.4.

Change-Id: Icccfb347a27ed0f2ff9a706d42b31f6a9d102fce
Signed-off-by: Allon Mureinik <amureini@redhat.com>