Merge pull request WordPress#4429 from WordPress/fix/gutenberg-activa…

…tion-undefined-constant-warning

Fix warning plugin activation

Update plugin version to 1.9.1

Update version to 1.9.0. (WordPress#3874)

Update plugin version to 1.8.1. (WordPress#3720)

Update version to 1.8. (WordPress#3694)

Bump version to 1.7. (WordPress#3479)

Bump version to 1.6.1. (WordPress#3309)

Bug fixes.

Update version to 1.6. (WordPress#3254)

Bump version to 1.5.2

Bump version to 1.5.1. (WordPress#3123)