Bump minimum Solidity version to 0.5.7 (#1724)

* Bump Solidity version to 0.5.7

* Add changelog entry.

contracts/drafts/Counters.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.7;
 
 import "../math/SafeMath.sol";
 

contracts/drafts/ERC1046/ERC20Metadata.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.7;
 
 import "../../token/ERC20/IERC20.sol";
 

contracts/drafts/ERC1820Implementer.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.7;
 
 import "./IERC1820Implementer.sol";
 

contracts/drafts/ERC20Migrator.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.7;
 
 import "../token/ERC20/IERC20.sol";
 import "../token/ERC20/ERC20Mintable.sol";

contracts/drafts/ERC20Snapshot.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.7;
 
 import "../math/SafeMath.sol";
 import "../utils/Arrays.sol";

contracts/drafts/ERC777/ERC777.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.7;
 
 import "./IERC777.sol";
 import "./IERC777Recipient.sol";

contracts/drafts/ERC777/IERC777.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.7;
 
 /**
  * @title ERC777 token interface

contracts/drafts/ERC777/IERC777Recipient.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.7;
 
 /**
  * @title ERC777 token recipient interface

contracts/drafts/ERC777/IERC777Sender.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.7;
 
 /**
  * @title ERC777 token sender interface

contracts/drafts/IERC1820Implementer.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.7;
 
 /**
  * @title IERC1820Implementer

contracts/drafts/IERC1820Registry.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.7;
 
 /**
  * @title ERC1820 Pseudo-introspection Registry Contract

contracts/drafts/SignatureBouncer.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.7;
 
 import "../access/roles/SignerRole.sol";
 import "../cryptography/ECDSA.sol";

contracts/drafts/SignedSafeMath.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.7;
 
 /**
  * @title SignedSafeMath

contracts/drafts/TokenVesting.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.7;
 
 import "../token/ERC20/SafeERC20.sol";
 import "../ownership/Ownable.sol";