Functions in interfaces changed to "external" (#1263)

contracts/token/ERC20/IERC20.sol

@@ -6,20 +6,20 @@ pragma solidity ^0.4.24;
  * @dev see https://github.com/ethereum/EIPs/issues/20
  */
 interface IERC20 {
-  function totalSupply() public view returns (uint256);
+  function totalSupply() external view returns (uint256);
 
-  function balanceOf(address _who) public view returns (uint256);
+  function balanceOf(address _who) external view returns (uint256);
 
   function allowance(address _owner, address _spender)
-    public view returns (uint256);
+    external view returns (uint256);
 
-  function transfer(address _to, uint256 _value) public returns (bool);
+  function transfer(address _to, uint256 _value) external returns (bool);
 
   function approve(address _spender, uint256 _value)
-    public returns (bool);
+    external returns (bool);
 
   function transferFrom(address _from, address _to, uint256 _value)
-    public returns (bool);
+    external returns (bool);
 
   event Transfer(
     address indexed from,