message: bump version and specify node/npm versions
diff: {
  "name": "blockstack",
  - "version": "0.8.2",
  + "version": "0.8.3",
  "description": "The Blockstack Javascript library for identity and authentication.",
  "main": "lib/index",
  "scripts": {
  "uuid": "^3.0.1",
  "validator": "^7.0.0",
  "zone-file": "^0.2.2"
  + },
  + "engines": {
  + "node": ">=6",
  + "npm": ">=5"
  }
  }
  