Fix postgraphile websocket/subscription omission path

[jonathanpeterwu]

Description

On build for the lambda example webpack bundling fails on omission of subscriptions.js as the npm install path has changed

• [x] My code matches the project's code style and yarn lint:fix passes.

[benjie]

Please also update the yarn lock (assuming an upgrade is required to trigger the issue?)

[jonathanpeterwu]

Updated with package upgrade

[SnowballAntrobus]

@benjie is there a reason in particular we would not want to update to the most recent version of postgraphile?

[benjie]

No; just that I've tested this on older versions and I don't fancy testing it again because working with Lambda is a PITA. I'd rather go with a "known good" version.