chore(package.json): use "files" field i.s.o. .npmignore

Author: chimurai

.npmignore

@@ -3,6 +3,10 @@
   "version": "0.17.1",
   "description": "The one-liner node.js proxy middleware for connect, express and browser-sync",
   "main": "index.js",
+  "files": [
+    "index.js",
+    "lib"
+  ],
   "scripts": {
     "clean": "rm -rf coverage",
     "test": "mocha --recursive --colors --reporter spec",