Shortcut for ShippedIssue#magazine_issue

Here we can write `ShippedIssue.magazine_issue` rather than always needing to write `ShippedIssue.issue.magazine_issue`, similar to the way that Spree uses  `LineItem.product` as a shortcut for `LineItem.variant.product`.
Merge pull request #36 from brchristian/patch-1

Shortcut for ShippedIssue#magazine_issue