travis: fix building on s390x

The travis build for s390x started to fail with:

Failed to fetch http://security.debian.org/debian-security/dists/jessie/updates/InRelease Unable to find expected entry 'main/binary-s390x/Packages' in Release file (Wrong sources.list entry or malformed file)

This changes the repository definition just like it is done for ppc64le.
Signed-off-by: Adrian Reber <areber@redhat.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

scripts/build/Dockerfile.s390x.hdr

@@ -2,3 +2,5 @@ FROM s390x/debian:jessie-backports
 
 ENV QEMU_CPU z900
 COPY scripts/build/qemu-user-static/usr/bin/qemu-s390x-static /usr/bin/qemu-s390x-static
+# The security repository does not seem to exist anymore
+RUN sed -i '/security/ d' /etc/apt/sources.list