Only publish the required files

Details: https://docs.npmjs.com/files/package.json#files

Author: benjie

Diff for: package.json

@@ -30,5 +30,9 @@
     "chai": "^4.1.1",
     "istanbul": "^0.4.5",
     "mocha": "^3.5.0"
-  }
+  },
+  "files": [
+    "index.js",
+    "index.d.ts"
+  ]
 }