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/utils/Pausable.sol

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: MIT
 
-pragma solidity ^0.6.0;
+pragma solidity ^0.7.0;
 
 import "../GSN/Context.sol";
 
@@ -29,7 +29,7 @@ contract Pausable is Context {
     /**
      * @dev Initializes the contract in unpaused state.
      */
-    constructor () internal {
+    constructor () {
         _paused = false;
     }