Symbol Instances Go Missing When Rendering Plugin
This issue is the same one that was described here: https://github.com/airbnb/react-sketchapp/pull/53#issuecomment-306266892

**Expected behavior:**
Symbols that have been updated, but their name has not changed should not break references to existing instances of that symbol.

**Observed behavior:**
Making changes to a symbol will cause instantiations of that symbol to go missing.

**How to reproduce:**

1. generate a symbol from react-sketchapp
2. put the symbol that is now in sketch on a page
3. update the component in code
4. npm run render picks up the change and pushes the change to sketch
5. the symbol you put on the page in step 2 now is "missing"

**Sketch version:**
43

**Please attach screenshots, a zip file of your project, and/or a link to your github project**
Example Code:
```
import { View, Text, makeSymbol, injectSymbols } from 'react-sketchapp'
import React from 'react'

const H1Container = () => <View><Text>Some other Text</Text></View>
H1Container.displayName = 'H1'

makeSymbol(H1Container)

export default context => {
  injectSymbols(context)
}
```
