Fix for change in thanks page handling by new ver

In WooCommerce 2.1, getting links for "thanks" page was changed.  This fix checks the versioning and uses the new method to remain backwards compatible with users running the plugin in old versions.

See: http://develop.woothemes.com/woocommerce/2013/11/woocommerce-2-1-beta-1-is-ready/
Update README.md
Merge pull request #11 from ionux/master

Fix for change in thanks page handling by new version of WC