[build] run copy-metafiles script before publishing only

The copy-metafiles script breaks installation on windows. It is actually only
needed before publishing. Thus we run it as a `prepublishOnly` script instead of
`prepublish` (for more info see https://docs.npmjs.com/misc/scripts#description)

package.json

@@ -20,7 +20,6 @@
   "scripts": {
     "prebuild": "rimraf lib",
     "build": "babel --quiet --out-dir lib src",
-    "postbuild": "npm run copy-metafiles",
     "copy-metafiles": "for DIR in memo-parser resolvers/node resolvers/webpack utils; do cp LICENSE .npmrc \"${DIR}/\"; done",
     "watch": "npm run mocha -- --watch tests/src",
     "pretest": "linklocal",
@@ -31,6 +30,7 @@
     "test-compiled": "npm run prepublish && NODE_PATH=./lib mocha --compilers js:babel-register --recursive tests/src",
     "test-all": "npm test && for resolver in ./resolvers/*; do cd $resolver && npm test && cd ../..; done",
     "prepublish": "npm run build",
+    "prepublishOnly": "npm run copy-metafiles",
     "coveralls": "nyc report --reporter lcovonly && cat ./coverage/lcov.info | coveralls"
   },
   "repository": {