Fix eslint errors in CI

[ggetz]

The latest eslint version has breaking changes. Resolves eslint errors in CI by setting eslint to exactly version 5.12.1.

[cesium-concierge]

Thanks for the pull request @ggetz!

• ✔️ Signed CLA found.
• ❔ CHANGES.md was not updated.
  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.
• ❔ Unit tests were not updated.
  • Make sure you've updated tests to reflect your changes, added tests for any new code, and ran the code coverage tool.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.
• Works (or fails gracefully) in IE11.

[mramato]

Can you write up an issue to unpin this once https://github.com/BenoitZugmeyer/eslint-plugin-html/issues/ is addressed?

[ggetz]

Opened #7536

[mramato]

Thanks, I confirmed that the eslint stage passed in CI, so in the interest of time I'm merging this which should fix master.