Update Shipwire's tracking_companies to match others

Webgistix and AmazonMWS responses have `tracking_companies` set to a hash with array values, whereas Shipwire returns a hash with string values.

Relevant code:

- [Shipwire](https://github.com/Shopify/active_fulfillment/blob/master/lib/active_fulfillment/fulfillment/services/shipwire.rb#L227)
- [AmazonMWS](https://github.com/Shopify/active_fulfillment/blob/2e52c43b586d79af9fa543fde0487fc8001eef92/lib/active_fulfillment/fulfillment/services/amazon_mws.rb#L245)
- [Webgistix](https://github.com/Shopify/active_fulfillment/blob/2e52c43b586d79af9fa543fde0487fc8001eef92/lib/active_fulfillment/fulfillment/services/webgistix.rb#L316-L317)

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