Bump hash-validator to 0.7.1

[jnummelin]

fixes #2092

After

$ cat ~/code/stacks/volumes/ext-vols.yml 
stack: jussi/redis-ext-vol-no-name
description: Just a simple Redis stack with volume
version: 0.0.1
services:
  redis:
    image: redis:3.2-alpine
    command: redis-server --appendonly yes
    volumes:
      - redis-data:/data

volumes:
  redis-data:
    external: false

$ k stack install ~/code/stacks/volumes/ext-vols.yml 
YAML validation failed! Aborting.

- "/Users/jussi/code/stacks/volumes/ext-vols.yml":
  - volumes:
      redis-data:
        external: is not valid

[SpComb]

LGTM

volumes:
  data:
    external: false

YAML validation failed! Aborting.

- test/volumes-test.yml:
  - volumes:
      data:
        external: is not valid