Serverless 0.5.x compatibility

This plugin doesn't seem to work with latest serverless v0.5.0.

``` json
/node_modules/bluebird/js/release/async.js:49
        fn = function () { throw arg; };
                           ^

TypeError: Path must be a string. Received undefined
  at assertPath (path.js:8:11)
  at Object.posix.join (path.js:479:5)
```
