Make id non-nullable for json-api resource type def

Author: Ronni Persson

flow-typed/browser/json-api_vx.x.x.js

@@ -95,7 +95,7 @@ declare module 'json-api' {
   };
 
   declare type JSONAPIResource = {|
-    id?: string,
+    id: string,
     type: string,
     attributes?: JSONAPIAttributes,
     relationships?: JSONAPIRelationships,