Adapt VERSION file to reflect 1.5.1 version

Merge pull request wp-cli#4656 from wp-cli/update-to-1-5-0

Update to 1.5.0

Bump version to 1.4.1

Merge pull request wp-cli#4449 from wp-cli/release-1-4-0

Bump version to 1.4.0

Merge pull request wp-cli#4273 from wp-cli/bump-1-3-0

Bump version to 1.3.0

Merge pull request wp-cli#4135 from wp-cli/bump-1-2-1

Bump version to 1.2.1

Merge pull request wp-cli#4115 from wp-cli/bump-1-2-0

Bump version to 1.2.0

Merge pull request wp-cli#3789 from wp-cli/bump-1-1-0

Bump version to 1.1.0

Merge pull request wp-cli#3622 from wp-cli/bump-1-0-0

Bump stable version to 1.0.0

Merge pull request wp-cli#3493 from wp-cli/update-0-25-0

Bump version to 0.25.0