Fix code to not rely on stringifying the number returned from #/

* When requiring mathn, the Fixnum#/ and Bignum#/ methods are
  replaced with an implementation that returns a different value.
* Fix max_weight comparisons to use integers
Refactor Spree::Calculator::Shipping::ActiveSHipping::Base#get_max_weight

* The logic to figure out the maximum package weight is now simpler and
  more straight forward.
Remove unnecesssary code that present the packages from being split up

* There is already logic that prevents too-large packages to be shipped by
  breaking packages over a specific weight threshold into smaller packages
  under the threshold. This specific logic blocks that from being used
  and is unnecessary. Also the fact that it did not cause any spec failures
  leads me to believe there are no identified valid use cases for it, so the
  code should be removed until such use cases can be provided.
Update calculator to not calculate shipping if there is no shipping address