Fix: Bluebird console.warn message

[cullenjett]

While using Renderer#toStream() a console message from Bluebird is emitted regarding a promise created in src/consumers/node-stream.js that was not returned.

Repro: Start a server that streams a response using Renderer#toStream() and observe the logs in the console. An example application where this behavior is happening can be found here.

Versions:

• Rapscallion 2.1.13 (with Bluebird 3.5.0)
• React 15.6.1
• Node 8.2.1
• Express 4.15.3

Error Message: (node:22461) Warning: a promise was created in a handler at my-repo/node_modules/rapscallion/lib/consumers/node-stream.js:43:9 but was not returned from it, see http://goo.gl/rRqMUw

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Closes #119

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- `2017-07-31T03:38:01.138Z:effd109`: The pull request was created - `2017-07-31T03:40:38.862Z:effd109`: @cullenjett checked `I have read and will comply with the [contribution guidelines](https://github.com/FormidableLabs/rapscallion/blob/master/CONTRIBUTE.md).`. - `2017-07-31T03:40:40.679Z:effd109`: @cullenjett checked `I have read and will comply with the [code of conduct](https://github.com/FormidableLabs/rapscallion/blob/master/CONTRIBUTE.md).`. - `2017-07-31T03:40:43.871Z:effd109`: @cullenjett checked `All related documentation has been updated to reflect the changes made. _(required)_`. - `2017-07-31T03:40:46.897Z:effd109`: @cullenjett checked `My commit messages are cleaned up and ready to merge. _(required)_`. - `2017-07-31T03:40:48.086Z:effd109`: @cullenjett selected `patch` as the semantic version.

[ryan-roemer]

@divmain -- How are you feeling about this one? (LGTM)

Would close a recently opened issue.

[88kami88]

@cullenjett I can merge this if you update with the latest master, please. Thanks!

[88kami88]

Closed in favor of #124