Using the new overridable function

contracts/crowdsale/Crowdsale.sol

@@ -77,7 +77,7 @@ contract Crowdsale {
     uint256 weiAmount = msg.value;
 
     // calculate token amount to be created
-    uint256 tokens = weiAmount.mul(rate);
+    uint256 tokens = getTokenAmount(weiAmount);
 
     // update state
     weiRaised = weiRaised.add(weiAmount);