Attempting to deploy function will remove the layer.

For instance:

Service deployed using sls deploy
Without changing the deployed function code, deploy function sls deploy -f fooBar
Code won't change as Serverless will let us know Serverless: Code not changed. Skipping function deployment.
The layer however will get removed, and executing the function will fail to execute in runtime because the code does not exist there.
Attaching to the deploy:function:packageFunction hook solves this.

Opened #144 to resolve this