Simpler tax rate check which Closes #3813

@mikejolley can you please check this conflict resolution?
@ChromeOrange definitely needs some more testing on this branch

Conflicts:
	classes/class-wc-cart.php
Added Russian Rubles to PayPal valid currencies list
Merge pull request #3821 from corsonr/release-2-0-15

Added Russian Rubles to PayPal valid currencies list
Allow variations JS to be used on multiple products in the same form

Conflicts:
	assets/js/frontend/add-to-cart-variation.js
standard should be blank. Closes #3799.

@coenjacobs .15
Merge remote-tracking branch 'woo/release-2-0-15' into release-2-0-15-tivnet

Conflicts:
	classes/abstracts/abstract-wc-product.php