please add whmcs integration checks

Hi

I had an error the last two days where ordering a product would redirect the buyer to the main cart page.

I had this before

The plugin checks if it can wright to class-http but it should also check if the whmcs-integration cache dir is writable

i dont know why but sometimes the dir is not and then u will get this behaviour

letting us know what is the problem is a nice addon to the plugin