travis: mount tmpfs into /run for docker containers

Otherwise we will see warnings like these ones:
Warn  (criu/kerndat.c:805): Can't load /run/criu.kdat
Warn  (criu/net.c:2726): Unable to get a socket network namespace
Warn  (criu/kerndat.c:844): Can't keep kdat cache on non-tempfs
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

scripts/travis/Makefile

@@ -20,7 +20,7 @@ $(TARGETS):
 	echo 'DOCKER_OPTS="--storage-driver=devicemapper"' > /etc/default/docker
 	restart docker
 	$(MAKE) -C ../build $@$(target-suffix)
-	docker run --rm -it --privileged -v /lib/modules:/lib/modules criu-$@ ./scripts/travis/asan.sh $(ZDTM_OPTIONS)
+	docker run --rm -it --privileged -v /lib/modules:/lib/modules --tmpfs /run criu-$@ ./scripts/travis/asan.sh $(ZDTM_OPTIONS)
 
 docker-test:
 	./docker-test.sh