Rails 5.2: Bump ActiveShipping

[joshRpowell]

5.2 support added here https://github.com/Shopify/active_shipping/commit/9b04b51c570ba14a77eac86e30d90531b0134bb5

Needs to be updated here https://github.com/spree-contrib/spree_active_shipping/blob/b6e71bd4bf9ba42960aa52e46db17d224f9e5e11/spree_active_shipping.gemspec#L21

this looks like it will require a refactor due to:

BREAKING CHANGE: Use [shopify/measured] instead of shopify/quantified for dimensions and units.

https://github.com/Shopify/active_shipping/blob/master/CHANGELOG.md#v200

which can be found here: shopify/measured

[joshRpowell]

@damianlegawiec @bbonislawski any suggestions/thoughts on how to proceed? Is this something you'll want up to date for the Rails 5.2 bump?

I'll gladly work up a PR. Trying not to duplicate any existing efforts.