Symbols example not working
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**Expected behavior:**

https://github.com/airbnb/react-sketchapp/tree/1974091c696470138216a54611abd125d5828af9/examples/symbols

Looking at the code, a composite symbol is created that defaults to a profile image and a red square.  The overrides specify that the image should be replaced and the red square replaced with a blue square with modified text.

**Observed behavior:**

No overrides are performed.  The rendered document does show overrides for each component as expected in the layer inspector, and appropriate changes can be made manually.

NB: Additionally, the URL specified as the image override no longer appears to be valid.

Looking at the source for both RSA and the SA plugin library suggests that the overrides property is not an object, but an array with objects with a particular set of properties - I can't see if this is being translated somewhere - https://github.com/BohemianCoding/SketchAPI/blob/e374fa3720ee7615353c5fe91f4ee437d3b8015e/docs/api/SymbolInstance.md

**How to reproduce:**

Build and run the example as documented.

**Sketch version:**
48.2

**Please attach screenshots, a zip file of your project, and/or a link to your github project**

![sketch](https://user-images.githubusercontent.com/2016856/36473601-e45cd074-16ec-11e8-813f-f5ca31957c24.jpeg)
