Fix transaction voiding issues with MerchantWare integration

**Issue**
Authorizations could not be properly voided. We don't want to have to first capture the payment before making the void.
(See #520)

**Changes**
Use version 4 of MerchantWare's API for the void transaction. This version allows for transactions to be voided before being captured.
There are also some changes to the remote tests due to server changes on MerchantWare's side.

**Review**
@jduff 
@Soleone