Assert that completeTransactions() was called when the app launches.

[bizz84]

• Assert that completeTransactions() was called when the app launches.
• Update PaymentQueueControllerTests to enqueue purchase calls in the correct order.

Related issue: #287.

Discussion

This PR adds a runtime assertion to check that completeTransactions() was called when the app launches. The assertion is checked when calling purchaseProduct() or restorePurchases(), as these calls are most likely to yield undesired results without a prior call to completeTransactions().

A better way of doing this would be to transform the runtime check into a compile-time constraint.
This could be done by making the CompleteTransactions struct a non-optional parameter passed to the initializer of SwiftyStoreKit. Example:

let swiftyStoreKit = SwiftyStoreKit(CompleteTransactions(atomically: atomically, callback: { purchases in 
   // ...
})
// save swiftyStoreKit and use elsewhere

This would require SwiftyStoreKit to not be a singleton, which would be a big API-breaking change that affects all clients.

For the time being - I will keep this as a runtime check. Hopefully this is enough to prevent problems down the line in testing.