Correctly call super.transferableTokens

Fix #307

contracts/token/VestedToken.sol

@@ -109,7 +109,7 @@ contract VestedToken is StandardToken, LimitedTransferToken {
   function transferableTokens(address holder, uint64 time) constant public returns (uint256) {
     uint256 grantIndex = tokenGrantsCount(holder);
 
-    if (grantIndex == 0) return balanceOf(holder); // shortcut for holder without grants
+    if (grantIndex == 0) return super.transferableTokens(holder, time); // shortcut for holder without grants
 
     // Iterate through all the grants the holder has, and add all non-vested tokens
     uint256 nonVested = 0;