fix build script

Author: Takeharu Oshida

package.json

@@ -10,7 +10,7 @@
     "test:local": "karma start",
     "test": "./node_modules/.bin/karma start --browsers Firefox --single-run",
     "clean": "rimraf dist",
-    "build": "clean & webpack"
+    "build": "npm run clean & webpack"
   },
   "repository": {
     "type": "git",