NotificationARNs in Serverless::Application don't accept !Ref

[keetonian]

Description:

You should be able to !Ref a SNS topic

Steps to reproduce the issue:

1. Create a Serverless::Application that !Refs a Sns topic
2. Try deploying or transforming the template

Observed result:
Error
Expected result:
Should deploy/transform

[ShreyaGangishetty]

Closing this issue as v1.15.0 is released