fixed an issue in versioning.mk (helm#3653)

The MUTABLE_IMAGE_RUDDER variables was using the DOCKER_VERSION instead of MUTABLE_VERSION
```
MUTABLE_IMAGE        := ${DOCKER_REGISTRY}/${IMAGE_PREFIX}/${SHORT_NAME}:${MUTABLE_VERSION}
MUTABLE_IMAGE_RUDDER := ${DOCKER_REGISTRY}/${IMAGE_PREFIX}/${SHORT_NAME_RUDDER}:${DOCKER_VERSION}
```

versioning.mk

@@ -28,7 +28,7 @@ LDFLAGS += -X k8s.io/helm/pkg/version.GitTreeState=${GIT_DIRTY}
 IMAGE                := ${DOCKER_REGISTRY}/${IMAGE_PREFIX}/${SHORT_NAME}:${DOCKER_VERSION}
 IMAGE_RUDDER         := ${DOCKER_REGISTRY}/${IMAGE_PREFIX}/${SHORT_NAME_RUDDER}:${DOCKER_VERSION}
 MUTABLE_IMAGE        := ${DOCKER_REGISTRY}/${IMAGE_PREFIX}/${SHORT_NAME}:${MUTABLE_VERSION}
-MUTABLE_IMAGE_RUDDER := ${DOCKER_REGISTRY}/${IMAGE_PREFIX}/${SHORT_NAME_RUDDER}:${DOCKER_VERSION}
+MUTABLE_IMAGE_RUDDER := ${DOCKER_REGISTRY}/${IMAGE_PREFIX}/${SHORT_NAME_RUDDER}:${MUTABLE_VERSION}
 
 DOCKER_PUSH = docker push
 ifeq ($(DOCKER_REGISTRY),gcr.io)