Update Action Scheduler to 2.2.4

[rrennick]

Fixes in this update

• Fatal Error in WC System Status when PHP < 7.0 PR #285
• Pin dependency wp-cli/wp-cli to 1.5.1 to match changes made in WooCommerce core: Pin dependency wp-cli/wp-cli to 1.5.1 woocommerce#23028
• PHP 5.2 compatibility PR #268
• Eliminate "non well formed numeric value" error PR #269

@james-allan I was getting a fatal error on the subtree command:

$ git subtree pull --prefix includes/libraries/action-scheduler https://github.com/Prospress/action-scheduler.git master
From https://github.com/Prospress/action-scheduler
 * branch              master     -> FETCH_HEAD
fatal: refusing to merge unrelated histories

Also, should the docs folder be removed?

[rrennick]

Sorry, create this in the wrong repo.