use new style hook if ver 2.1.20+

[BlueGreenMagick]

Description

Resolves #13

Checklist:

Please replace the space inside the brackets with an x and fill out the ellipses if the following items apply:

• [ ] I've read and understood the contribution guidelines
• [ ] I've tested my changes against at least one of the following Anki builds:
  • [ ] Latest standard Anki 2.1 binary build [required for Anki-compatible 2.1 add-ons]
  • [ ] Latest alternative Anki 2.1 binary build
  • [ ] Latest Anki 2.0 binary build [required for Anki 2.0-compatible add-ons]
• [ ] I've tested my changes on at least one of the following platforms:
  • [x] Linux, version:
  • [ ] Windows, version:
  • [ ] macOS, version:
• [ ] My changes potentially affect non-desktop platforms, of which I've tested:
  • [ ] AnkiMobile, version:
  • [ ] AnkiDroid, version:
  • [ ] AnkiWeb

[glutanimate]

Looks good, thanks!