Merge remote-tracking branch 'upstream/master' into zipfiles

Changes: https://github.com/wp-cli/wp-cli/compare/fbaab715...498fb6

behat: replace 'a large image file' step with a more generic and explicit 'download' step
behat: Convert duplicate plugin/theme tests to scenario outlines

Converts the plugin/theme tests added in e749f71 to Behat's scenario outlines (http://docs.behat.org/guides/1.gherkin.html#scenario-outlines), which removes test duplication.

vendor/bin/behat features/upgradables.feature --expand