Migrate contracts to Solidity 0.7 (#2319)

* Update contract pragmas to solidity 0.7

* Remove internal declaration on constructors

* Reference SafeMath explicitely

* Remove public constructor declaration from abstract contracts

* Remove public constructor declaration from non-abstract contracts

contracts/payment/escrow/ConditionalEscrow.sol

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: MIT
 
-pragma solidity ^0.6.0;
+pragma solidity ^0.7.0;
 
 import "./Escrow.sol";
 

contracts/payment/escrow/Escrow.sol

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: MIT
 
-pragma solidity ^0.6.0;
+pragma solidity ^0.7.0;
 
 import "../../math/SafeMath.sol";
 import "../../access/Ownable.sol";

contracts/payment/escrow/RefundEscrow.sol

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: MIT
 
-pragma solidity ^0.6.0;
+pragma solidity ^0.7.0;
 
 import "./ConditionalEscrow.sol";
 
@@ -27,7 +27,7 @@ contract RefundEscrow is ConditionalEscrow {
      * @dev Constructor.
      * @param beneficiary The beneficiary of the deposits.
      */
-    constructor (address payable beneficiary) public {
+    constructor (address payable beneficiary) {
         require(beneficiary != address(0), "RefundEscrow: beneficiary is the zero address");
         _beneficiary = beneficiary;
         _state = State.Active;