Remove prepublishOnly npm script

It's broken in npm@<5

Author: xzyfer

package.json

@@ -6,8 +6,7 @@
   "scripts": {
     "test": "mocha",
     "clean": "rm lib/*",
-    "build": "npm run clean; babel src/ --out-dir lib",
-    "prepublishOnly": "npm run build"
+    "build": "npm run clean; babel src/ --out-dir lib"
   },
   "files": [
     "index.js",