Change refs in one pass, making switching refs atomic

[LPGhatguy]

This is a minor change that ideally shouldn't be user-visible.

Before #92, there was a bug in Roact where when refs were updated, nothing would happen. There was a typo in the reconciler code that resulted in the comparison in the reconciler always determining that refs never changed!

After #92, an issue was unveiled by @ZoteTheMighty via some internal Roact code that means that with the typical use of a ref:

[Roact.Ref] = (rbx)
   self.foo = rbx
end

...there is a period of time where self.foo will be nil when Roact nullifies the old ref, makes changes, and then passes the same object to the new ref, since by function identity it changed. This doesn't affect code that keeps a stable ref function, and is only visible to code that acts on a ref in response to a change event.

Checklist before submitting:

• Added entry to CHANGELOG.md
• Tests added and updated I don't think we can test this

[coveralls]

Coverage decreased (-0.03%) to 90.382% when pulling 2fd8db4 on atomic-refs into f8edb9f on master.

[LPGhatguy]

Coverage drop is in... PureComponent?

[ZoteTheMighty]

Coveralls seems to have gotten very confused about what actually changed...

[ZoteTheMighty]

Can confirm that the edge case we saw is resolved by this. Looks good so far, I'll take a look and approve once there's a test that looks good.