CI x86_64: switch to Zesty, add clang build

The x86_64 job is the one we run inside Docker (as opposed to 'local',
which runs on the host). Let's update this to use Ubuntu Zesty, and
since it has clang-4 (unlike the 'local' one) it makes sense to add
a clang build.
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

.travis.yml

@@ -10,6 +10,7 @@ env:
   - TR_ARCH=alpine
   - TR_ARCH=asan
   - TR_ARCH=x86_64
+  - TR_ARCH=x86_64      CLANG=1
   - TR_ARCH=armv7hf
   - TR_ARCH=aarch64
   - TR_ARCH=ppc64le

scripts/build/Dockerfile.x86_64.hdr

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