core: use slf4j-jdk14 1.5.6

Downgraded slf4j-jdk14 to be compatible with log4j 1.2.16 that we use in
the project.

Change-Id: I0b3d1363da413881ee1aa89738dd8d7ea6ac52c5
Signed-off-by: Allon Mureinik <amureini@redhat.com>