pom: enforce maven-release-plugin 2.4.2

the latest plugin broke git support.

Acked-by: Gerd Behrmann
Target: master, 2.7, 2.6
Require-book: no
Require-notes: no
(cherry picked from commit 5a5ccd2d5c3353331881dc5df0645a429df836e9)
Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Merge pull request #390 from kofemann/fix-git-2.6

pom: enforce maven-release-plugin 2.4.2