#3318 Update Node version on Glitch demos

[Anonymous]

Originally created by: kevinfarrugia

Library Affected:
All workbox demos on Glitch. For example https://glitch.com/~workbox-core

Browser & Platform:
All browsers

Issue or Feature Request Description:
The demos hosted on Glitch are no longer working because Glitch supports Node 12.0 upwards. Please update the Node JS version in the project's package.json.