Use ActionDispatch::Request#remote_ip instead of reinvent the wheel

http://apidock.com/rails/ActionDispatch/Request/remote_ip

Fixes #4490
Fix issue changing between payment methods in admin

Fixes #4358
Fixes #4203
Payment credit_allowed deducts from offsets

fixes #4403
Fixes #4407
ShippingMethod#shipments should look up shipments through linked shipping rates

It used to be that shipments linked directly to shipping methods, but now the relationship is done through shipping rates.

Fixes #4492
Bump to 2.0.10