convert ERC721Pausable to initializers

contracts/mocks/ERC721PausableMock.sol

 pragma solidity ^0.4.24;
 
+import "../Initializable.sol";
 import "../token/ERC721/ERC721Pausable.sol";
 import "./PauserRoleMock.sol";
 
@@ -8,7 +9,11 @@ import "./PauserRoleMock.sol";
  * @title ERC721PausableMock
  * This mock just provides a public mint, burn and exists functions for testing purposes
  */
-contract ERC721PausableMock is ERC721Pausable, PauserRoleMock {
+contract ERC721PausableMock is Initializable, ERC721Pausable, PauserRoleMock {
+  constructor() {
+    ERC721Pausable.initialize();
+  }
+
   function mint(address to, uint256 tokenId) public {
     super._mint(to, tokenId);
   }

contracts/token/ERC721/ERC721Pausable.sol

 pragma solidity ^0.4.24;
 
+import "../../Initializable.sol";
 import "./ERC721.sol";
 import "../../lifecycle/Pausable.sol";
 
@@ -8,7 +9,12 @@ import "../../lifecycle/Pausable.sol";
  * @title ERC721 Non-Fungible Pausable token
  * @dev ERC721 modified with pausable transfers.
  **/
-contract ERC721Pausable is ERC721, Pausable {
+contract ERC721Pausable is Initializable, ERC721, Pausable {
+  function initialize() public initializer {
+    ERC721.initialize();
+    Pausable.initialize();
+  }
+
   function approve(
     address to,
     uint256 tokenId