Bump GitHub Actions runners

[HonkingGoose]

Changes:

• Bump actions/checkout to version 2.3.3
• Bump actions/setup-node to latest stable v1.1.4 release (v2 is in beta)
• Bump actions/cache to latest version 2.1.1. (Main benefit should be "Increased performance and improved cache sizes using zstd for compression for Linux and macOS runners", according to the actions/cache v2.0.0 changelog)

Context:

You're using outdated GitHub action runners. This pull request updates those to the latest stable releases.
You can verify that the new actions still work, because this pull request uses the updated versions.

References:

https://github.com/actions/checkout/releases/tag/v2.3.3
https://github.com/actions/setup-node/releases/tag/v1.4.4
https://github.com/actions/cache/releases/tag/v2.1.1

Dependabot can check GitHub actions (want a pull request?)

By the way, Dependabot can check for github-actions updates, if you want to have the Dependabot check for GitHub actions update in future, let me know and I'll write the pull request.

[adamwathan]

Thanks!