fixes gh-99 PullPaymentMock needed a payable constructor

contracts/test-helpers/PullPaymentMock.sol

@@ -3,6 +3,9 @@ import '../PullPayment.sol';
 
 // mock class using PullPayment
 contract PullPaymentMock is PullPayment {
+
+  function PullPaymentMock() payable { }
+
   // test helper function to call asyncSend
   function callSend(address dest, uint amount) {
     asyncSend(dest, amount);