Update to Request-Promise v0.3.1

Hi @mjw56 

Out of curiosity I checked your code to see if you are affected by braking changes when upgrading to Request-Promise 0.3.1.

Actually you are. So I added the `.promise()` call to ensure full compatibility. You can find the details in the [migration instructions](https://github.com/tyabonil/request-promise#migrating-from-02x-to-03x).

Cheers,
Nico