Added a toString() method to DuoCredentials
Switch the order that the flow/request scopes are checked for the TGT

The initial version checked the flowScope, then the requestScope for
the TGTid.  This caused a problem because when a user re-authenticates,
they have TGTids in both scopes: the new TGT in the request and the old
one in the flow.
Increase version for hot fix