Travis: check mrproper cleanings

Let there be more make-tests.

Cc: Adrian Reber <adrian@lisas.de>
Cc: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

scripts/build/Dockerfile.tmpl

@@ -22,3 +22,5 @@ COPY . /criu
 WORKDIR /criu
 
 RUN make clean && make -j $(nproc)
+RUN make mrproper
+RUN bash -c 'CLEAN="$(git clean -ndx --exclude=scripts/build)"; echo "${CLEAN}"; test -z "${CLEAN}"; exit $?'