change SwiftyJSON dependency to 2.3.1 in readme

isExists() for JSON has not been add upon SwiftyJSON 2.3.1, and is renamed to exists() in this later PR: SwiftyJSON/SwiftyJSON#457.

Specifying "~> 2.3.0" might not work, and let users run into the following error:

```
error: value of type 'JSON' has no member 'isExists'
```

Readme.md

@@ -31,7 +31,7 @@ The aim of this library is to provide reusable React Native components that can
   use_frameworks!
 
   target 'MyApp' do
-    pod 'SwiftyJSON', '~> 2.3'
+    pod 'SwiftyJSON', '2.3.1'
     pod 'Charts', '2.2.3'
   end
   ```