convert ERC721Burnable to initializers

contracts/mocks/ERC721MintableBurnableImpl.sol

@@ -17,5 +17,6 @@ contract ERC721MintableBurnableImpl
   {
     ERC721Full.initialize("Test", "TEST");
     ERC721Mintable.initialize();
+    ERC721Burnable.initialize();
   }
 }

contracts/token/ERC721/ERC721Burnable.sol

@@ -1,9 +1,14 @@
 pragma solidity ^0.4.24;
 
+import "../../Initializable.sol";
 import "./ERC721.sol";
 
 
-contract ERC721Burnable is ERC721 {
+contract ERC721Burnable is Initializable, ERC721 {
+  function initialize() public initializer {
+    ERC721.initialize();
+  }
+
   function burn(uint256 tokenId)
     public
   {