chore(files): Explicitly list our package files

Author: iarna

package.json

@@ -42,5 +42,9 @@
     "tap": "^10.7.2",
     "weallbehave": "^1.2.0",
     "weallcontribute": "^1.0.8"
-  }
+  },
+  "files": [
+    "index.js",
+    "lib/spawn.js"
+  ]
 }