[fixed] Eliminate unnecessary renders when modal is closed

[fuller]

Fixes #[issue number].

Changes proposed:

• Early return if the modal is closed and will remain closed. This prevents us from doing additional DOM check and running renderPortal again unnecessarily. This was my original intention with [fixed] Multiple modals on page causing contradictory class updates #328, but is greatly simplified with v2 code.

I wasn't exactly sure if we wanted another test as all current tests are passing and this one (https://github.com/reactjs/react-modal/blob/master/specs/Modal.spec.js#L233) covers the scenario of multiple modals rendering.

Upgrade Path (for changed or removed APIs):

Acceptance Checklist:

• All commits have been squashed to one.
• The commit message follows the guidelines in CONTRIBUTING.md.
• Documentation (README.md) and examples have been updated as needed.
• If this is a code change, a spec testing the functionality has been added.
• If the commit message has [changed] or [removed], there is an upgrade path above.

[coveralls]

Coverage remained the same at 71.429% when pulling 430aa83 on ajfuller:fix/unnecessary-renders into 6c780ae on reactjs:master.

[coveralls]

Coverage remained the same at 71.429% when pulling 430aa83 on ajfuller:fix/unnecessary-renders into 6c780ae on reactjs:master.

[diasbruno]

Released v2.0.6.