Use sql rather than zone.include? method to find matches.

In applications with a large number of tax zones recalculating tax is
extremely slow.  In my particular instance transitioning form the
address to delivery step of checkout was taking up to 15 seconds, and
this change cut that in half.

Transition from Address to Delivery - Before:
Redirected to /checkout/delivery
Completed 302 Found in 14107ms (ActiveRecord: 252.5ms | Elasticsearch:
0.0ms)
Completed 302 Found in 13524ms (ActiveRecord: 195.2ms | Elasticsearch:
0.0ms)

Transition from Address to Delivery - After:
Redirected to /checkout/delivery
Completed 302 Found in 5423ms (ActiveRecord: 178.4ms | Elasticsearch:
0.0ms)
Completed 302 Found in 6223ms (ActiveRecord: 212.2ms | Elasticsearch:
0.0ms)

I’d like to figure out how to improve this even more, but cutting time
in half with this small change is a good start.
Remove Order#exclude_tax? as it is not used anywhere.
Scope tax rates to the orders tax zone.  Cuts my address to delivery transition from 6 seconds to 1 second
Removing bigdecimal/util require that was needed for ruby 1.8 support

Fixes #4551