travis-ci: run compel tests on build stage

It took a little time on travis (I didn't measure, just starred
at build and it added not visible delay for build).
But it will help to verify that compel works on all supported arches.

Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Reviewed-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

scripts/build/Dockerfile.tmpl

@@ -27,3 +27,4 @@ RUN make -j $(nproc) CC=$CC V=1
 RUN make mrproper
 RUN bash -c 'CLEAN="$(git clean -ndx --exclude=scripts/build --exclude=.config)"; echo "${CLEAN}"; test -z "${CLEAN}"; exit $?'
 RUN make -j $(nproc) CC=$CC -C test/zdtm
+RUN make test/compel/handle_binary && ./test/compel/handle_binary