Test latest version of WP, now 4.0
Mimic core's behavior of preventing orphaned terms for all versions
Update orphan term test to get around core's new check for parent
Merge pull request #1381 from wp-cli/require-wp-version

Tag syntax for requiring a specific min version of WP
Merge pull request #1384 from wp-cli/test-40

Test latest version of WP, now 4.0
Merge branch 'master' into lang-command