travis: use xenial instead of zesty

xenial is LTS and it is more stable and popular.
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

scripts/build/Dockerfile.aarch64.hdr

-FROM arm64v8/ubuntu:zesty
+FROM arm64v8/ubuntu:xenial
 
 COPY scripts/build/qemu-user-static/usr/bin/qemu-aarch64-static /usr/bin/qemu-aarch64-static

scripts/build/Dockerfile.armv7hf.hdr

-FROM arm32v7/ubuntu:zesty
+FROM arm32v7/ubuntu:xenial
 
 COPY scripts/build/qemu-user-static/usr/bin/qemu-arm-static /usr/bin/qemu-arm-static

scripts/build/Dockerfile.ppc64le.hdr

-FROM ppc64le/ubuntu:zesty
+FROM ppc64le/ubuntu:xenial
 
 ENV QEMU_CPU POWER8
 COPY scripts/build/qemu-user-static/usr/bin/qemu-ppc64le-static /usr/bin/qemu-ppc64le-static

scripts/build/Dockerfile.x86_64.hdr

-FROM ubuntu:zesty
+FROM ubuntu:xenial
 
 RUN apt-get update -qq && apt-get install -qq \
                 gcc-multilib