check Node.js when npm run to kindly notice developers who are using node version below v6.9 (#159)

package.json

@@ -18,9 +18,9 @@
     "ts-node": "ts-node",
     "dist": "npm run clean && tsc && shx cp package.json dist/ && shx cp src/puppet-web/*.js dist/src/puppet-web/",
     "coverage": "nyc report --reporter=text-lcov | coveralls",
-    "doctor": "ts-node bin/doctor",
+    "doctor": "check-node-version --node '>= 6.9' && ts-node bin/doctor",
     "clean": "shx rm -fr dist/*",
-    "lint": "npm run eslint && npm run tslint",
+    "lint": "check-node-version --node '>= 6.9' && npm run eslint && npm run tslint",
     "nycava": "WECHATY_HEAD=chrome nyc ava --serial --fail-fast --verbose --timeout=3m \"dist/{src,test}/**/*.spec.js\"",
     "eslint": "eslint \"{bin,example,src,test}/**/*.js\" --ignore-pattern=\"test/fixture/**\"",
     "tslint": "tslint \"{bin,example,src,test}/**/*.ts\" --exclude=\"test/fixture/**\" --exclude=\"dist/\" && tsc --noEmit",
@@ -125,6 +125,7 @@
     "apiai": "3.0.3",
     "ava": "0.17.0",
     "babel-eslint": "7.1.1",
+    "check-node-version": "^1.1.2",
     "cookie-parser": "1.4.3",
     "coveralls": "2.11.14",
     "cross-env": "3.1.3",