Drop test for term with non-existent parent

Core's term API has changed, and no longer will permit the
pre-condition. Our code still works; we just don't need to test for it
anymore.
Explicitly install WordPress importer

This will let us catch plugin installation failures, instead of letting
the precondition pass and test fail later
Use our formatter, so we can catch errors
Update version check test to accommodate WP4.1
Use Travis' nifty dependency caching
Merge pull request #1575 from wp-cli/1493-explict-install-export

Update tests for WordPress 4.1
Merge branch 'master' into 1493-check-install-failure