Add missing "unrecognized" enum to GatewayRejectionReason

Add new GatewayRejectionReason "unrecognized".

The BrainTree Server is now returning a new "GatewayRejectionReason" called "unrecognized" is not being
handled by the latest java braintree api.

java.lang.IllegalArgumentException: No enum const class com.braintreegateway.Transaction$GatewayRejectionReason.UNRECOGNIZED
        at java.lang.Enum.valueOf(Enum.java:214) ~[na:1.6.0_24]
        at com.braintreegateway.util.EnumUtils.findByName(EnumUtils.java:12) ~[braintree-java-2.24.0.jar:na]
        at com.braintreegateway.Transaction.<init>(Transaction.java:155) ~[braintree-java-2.24.0.jar:na]