Dockerfile.x86_64: use Ubuntu Trusty

Let's use Ubuntu Trusty Tahr (aka 14.04) for x86_64 QEMU build, as
1. this is the version that is used for other QEMU arches;
2. Ubuntu Wily, used before, is EOL as of July 28, 2016.

travis-ci: success for QEMU+Dockerfile improvements + clang build
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Acked-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

scripts/build/Dockerfile.x86_64.hdr

-FROM ubuntu:wily
+FROM ubuntu:trusty