fix(Modal): Incorrect display location of Modal with CSS 2.3.1

[mgandley]

Steps to Reproduce

1. Use a modal.

Expected
The modal should be centered.

Result
The modal is not centered.

Testcase
Working: https://codepen.io/anon/pen/qopZBP
Not Working: https://codepen.io/anon/pen/qopZBP

Same code, only difference is using css 2.2.4 vs using css 2.3.1.

Not sure if this is an issue with the react library or the CSS, opening issue in both locations.

[welcome]

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[brianespinosa]

Duplicate of #2550

[mgandley]

Derp. #2250
Beaten by seconds. Thanks @brianespinosa.