[chore] upgrade React devDeps to 15.6

[fuller]

Fixes #[issue number].

Changes proposed:

• Upgrade React to the latest version
• Remove the deprecated react-addons-test-utils in favor of accessing from react-dom/test-utils

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 84.821% when pulling dee03b7 on ajfuller:upgrade/react-dep into 88d2ae1 on reactjs:master.

[diasbruno]

This PR has some changes that are out of the scope, but refactoring is always a good thing.

Some of the changes makes lines longer. You can move some of the expect(CHECK) to a variable, like:

const check = ...;
expect(check).toBe(...);

jsx syntax is better to read if they have their own line, and, sometimes, wrapped with (), so any editor can indent better/correct. Example:

ReactDOM.render((
  <Modal isOpen appElement={el} />
), node);

[diasbruno]

Now, we can use ... operator instead of Object.assign.

[diasbruno]

Now, we can use { key, keyCode: code, which: code }.

[diasbruno]

...or change the code on the arguments to keyCode.
{ key, keyCode, which: keyCode }

[diasbruno]

@ajfuller I'll finish to update the changelog and I'll merge this PR, then we can release a version.

[fuller]

Sorry I have prettier enabled in Atom so it made a few more changes than I had expected. I'll make the requested changes.

[fuller]

@diasbruno I've cleaned this up so it's just the upgrade related changes. Other refactoring can be in a separate PR 😸.

[coveralls]

Coverage remained the same at 84.821% when pulling c8b5fcd on ajfuller:upgrade/react-dep into 8480042 on reactjs:master.

[coveralls]

Coverage remained the same at 84.821% when pulling c8b5fcd on ajfuller:upgrade/react-dep into 8480042 on reactjs:master.