Woocommerce Amazon S3 Storage 2.7.1

* Tweak - Updated the recommended privacy policy texts.
* Tweak - Adapted the database migration script for version 2.0.0 to the recent WooCommerce versions.
* Tweak - Added a database migration script to expire the region transients for versions lower than 2.1.5.
* Tweak - Tested compatibility with WC 7.4.
* Fix - Update the plugin version in the options table on activation.
* Dev - New plugin updater.
* Dev - Initialize the plugin when the main file is loaded.
* Dev - Updated the minimum requirements to WC 3.7, WP 4.9, and PHP 5.6

* Fix - Fixed incorrect download ID value when adding files with the S3 wizard.

* Tweak - Limit the maximum value of the 'URL expiration time' setting to seven days.
* Tweak - Tested compatibility with WC 7.3