fix(package): update npm to version 6.8.0

Author: greenkeeper[bot]

package.json

@@ -23,7 +23,7 @@
  "lodash": "^4.17.4",
  "nerf-dart": "^1.0.0",
  "normalize-url": "^4.0.0",
- "npm": "6.5.0",
+ "npm": "^6.8.0",
  "rc": "^1.2.8",
  "read-pkg": "^4.0.0",
  "registry-auth-token": "^3.3.1",