Fix: default to index.js if package.json doesn't specify

Author: Anand Thakker

lib/args.js

@@ -72,7 +72,7 @@ module.exports = function (args) {
   } else {
     try {
       var p = require(path.resolve('package.json'));
-      inputs = [p.main];
+      inputs = [p.main || 'index.js'];
       name = name || p.name;
       version = version || p.version;
       if (p.browserify && p.browserify.transform) {