Fix shipped?(issue) method

This corrects what appears to have been a bug in the `Subscription.shipped?(issue)` method, where the logic had been:

    !shipped_issues.where(:id => issue.id).empty?

which seemed off to me -- the `:id` of a shipped_issue would be the shipped_issue's _own_ id, not the id of the issue being checked for. So I changed that to

    !shipped_issues.where(issue: issue).empty?

which I think is the original intention.

Note that I also suggested changing `!empty?` to `present?` for simplicity:

    shipped_issues.where(issue: issue).present?
Merge pull request #44 from brchristian/patch-5

Fix shipped?(issue) method