use openzeppelin IERC20

contracts/ERC20Bucket.sol

@@ -2,7 +2,7 @@ pragma solidity ^0.6.1;
 pragma experimental ABIEncoderV2;
 
 import "./Bucket.sol";
-import "./erc20/IERC20.sol";
+import "@openzeppelin/contracts/token/ERC20/IERC20.sol";
 
 contract ERC20Bucket is Bucket {
   uint256 public redeemableSupply;

contracts/erc20/IERC20.sol

@@ -1,15 +0,0 @@
-pragma solidity ^0.6.1;
-
-// https://github.com/ethereum/EIPs/issues/20
-
-interface IERC20 {
-  event Transfer(address indexed _from, address indexed _to, uint256 _value);
-  event Approval(address indexed _owner, address indexed _spender, uint256 _value);
-
-  function transfer(address _to, uint256 _value) external returns (bool success);
-  function approve(address _spender, uint256 _value) external returns (bool success);
-  function transferFrom(address _from, address _to, uint256 _value) external returns (bool success);
-  function balanceOf(address _owner) external view returns (uint256 balance);
-  function allowance(address _owner, address _spender) external view returns (uint256 remaining);
-  function totalSupply() external view returns (uint256 supply);
-}

contracts/erc20/IERC20Detailed.sol

@@ -1,6 +1,6 @@
 pragma solidity >=0.5.0 <0.7.0;
 
-import "./IERC20.sol";
+import "@openzeppelin/contracts/token/ERC20/IERC20.sol";
 
 abstract contract IERC20Detailed is IERC20 {
   function name() virtual public view returns (string memory);

contracts/erc20/StandardToken.sol

@@ -1,6 +1,6 @@
 pragma solidity ^0.6.1;
 
-import "./IERC20.sol";
+import "@openzeppelin/contracts/token/ERC20/IERC20.sol";
 
 contract StandardToken is IERC20 {
 

package.json

@@ -0,0 +1,6 @@
+{
+  "dependencies": {
+    "@openzeppelin/contracts": "^3.2.0",
+    "eth-gas-reporter": "^0.2.17"
+  }
+}