fix: remove 'ref is not a prop' warning (#613)

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<!-- What changes are being made? (What feature/bug is being fixed here?) -->

**What**:
I've swapped the order of the operands in `validateGetRootPropsCalledCorrectly` to remove unnecessary warning from `react`.

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**Why**:
I'm messed up short-circuit logic in 3.1.3 and now getting this warning for composite components with `forwardRef` API.
![image](https://user-images.githubusercontent.com/15676655/47385043-9fc98480-d711-11e8-9e0b-4b4a55cdeb72.png)

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- [ ] Tests
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Author: akronb

src/downshift.js

@@ -1168,7 +1168,7 @@ function validateGetRootPropsCalledCorrectly(element, {refKey}) {
       `downshift: You returned a DOM element. You should not specify a refKey in getRootProps. You specified "${refKey}"`,
     )
   }
-  if (!getElementProps(element)[refKey] && !isForwardRef(element)) {
+  if (!isForwardRef(element) && !getElementProps(element)[refKey]) {
     // eslint-disable-next-line no-console
     console.error(
       `downshift: You must apply the ref prop "${refKey}" from getRootProps onto your root element.`,