adding prepare command for npm 4+, see http://blog.lholmquist.org/npm-prepublish-changes/

Author: kzap

package.json

@@ -49,6 +49,7 @@
     "coveralls": "nyc report --reporter=text-lcov | coveralls",
     "lint": "standard",
     "prepublish": "npm run build",
+    "prepare": "npm run build",
     "pretest": "npm run lint",
     "serve": "ss fixture/onmount/_docpress",
     "test": "nyc mocha",