Warning that step.id is null when the Wizard is first time rendering.

Hi,

First of all great library. Does what it says and does it best.
I was trying it out and was getting warning.
`Warning: Failed prop type: The prop `step.id` is marked as required in `Steps`, but its value is `null`. in Steps (created by Wizard) in Wizard (created by Onboarding)`

Turns out the same warning was replicated in the console.log of the [official examples](http://americanexpress.io/react-albus/) from add animation onwards.

Problem:
Basically whenever I try to render Step, using Wizard's render api, this warning shows up.
The default props of Steps.id = null. When the object is rendering it throws an error.

Can you please check?

Start by tracing the Steps component through the Wizard render API, then reproduce the warning in the official examples, especially from the animation example onward. Check how the required step.id prop is handled during the first render and verify that the warning no longer appears when rendering a Step through the render API.

Written by the indexing model from the issue text.