Deprecation fix: to saved_changes?

[joshRpowell]

Resolves:

DEPRECATION WARNING: The behavior of `changed?` inside of after callbacks will be changing in the next version of Rails. The new return value will ref
lect the behavior of calling the method after `save` returned (e.g. the opposite of what it returns now). To maintain the current behavior, use `saved
_changes?` instead. (called from update_inventory at /Users/jpowell/.gem/ruby/2.4.4/bundler/gems/spree-product-assembly-92ad5f4342a7/app/models/spree/
line_item_decorator.rb:43)

[joshRpowell]

CI fails on all Spree 3.2 which supports Rails 5.0. Green for Spree >= 3.3

Perhaps its worth versioning this Gem?