Bump minimum Solidity version to 0.5.7 (#1724)

* Bump Solidity version to 0.5.7

* Add changelog entry.

contracts/access/Roles.sol

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

contracts/access/roles/CapperRole.sol

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

contracts/access/roles/MinterRole.sol

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

contracts/access/roles/PauserRole.sol

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

contracts/access/roles/SignerRole.sol

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

contracts/access/roles/WhitelistAdminRole.sol

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

contracts/access/roles/WhitelistedRole.sol

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