IncPriceCrowdsale consistently returns 0 outside of the crowdsale window. (#1442)

(cherry picked from commit 9155bfe27f)

contracts/crowdsale/price/IncreasingPriceCrowdsale.sol

@@ -47,6 +47,10 @@ contract IncreasingPriceCrowdsale is TimedCrowdsale {
    * @return The number of tokens a buyer gets per wei at a given time
    */
   function getCurrentRate() public view returns (uint256) {
+    if (!isOpen()) {
+      return 0;
+    }
+
     // solium-disable-next-line security/no-block-members
     uint256 elapsedTime = block.timestamp.sub(openingTime());
     uint256 timeRange = closingTime().sub(openingTime());