Monotonic v3.0.9

• Upgrade Proof to 9.0.3.
• Build with Node.js 14 on Travis CI. Closes #30.
• Add clicklets to README.md. See #30.
• Build with Node.js 16 at Travis CI. See #30.
• Build with GitHub Actions. See #30.
• Update .gitignore. See #30.
• Update LICENSE for 2021. See #30.
• Fix Proof file suffix in package.json. See #29.
• Use Proof file suffix. Closes #29.
• Revert tests to Proof. See #29.

Closes #31.