ERC20 totalSupply changed from public property to a function (#666)

Fixes OpenZeppelin/zeppelin-solidity#434

contracts/token/BasicToken.sol

@@ -14,6 +14,15 @@ contract BasicToken is ERC20Basic {
 
   mapping(address => uint256) balances;
 
+  uint256 totalSupply_;
+
+  /**
+  * @dev total number of tokens in existence
+  */
+  function totalSupply() public view returns (uint256) {
+    return totalSupply_;
+  }
+
   /**
   * @dev transfer token for a specified address
   * @param _to The address to transfer to.