Server Cache Policy

[oruburos]

Change in Cache Policy

The line doesn't actually mean "do not use cache", but "revalidate the file with the server".

Fixes #1491

https://github.com/processing/p5.js-web-editor/pull/1491/files/78f87b6ec17c7546ecb841c870bccab59c70ca28#r453473436I

I have verified that this pull request:

• has no linting errors (npm run lint)
• is from a uniquely-named feature branch and has been rebased on top of the latest master. (If I was asked to make more changes, I have made sure to rebase onto master then too)
• is descriptively named and links to an issue number, i.e. Fixes #123

[catarak]

Thank you!!