chore(license): Update copyright notice to 2020 [ci skip]

[damingerdai]

No description provided.

[google-cla]

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[damingerdai]

@googlebot I signed it!

[johnjbarton]

Do you know if this is needed in some way?

[karmarunnerbot]

🎉 This PR is included in version 6.0.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀