Added missing initializations.

contracts/token/ERC721/ERC721MetadataMintable.sol

@@ -11,6 +11,7 @@ import "../../access/roles/MinterRole.sol";
  */
 contract ERC721MetadataMintable is Initializable, ERC721, ERC721Metadata, MinterRole {
   function initialize() public initializer {
+    ERC721.initialize();
     MinterRole.initialize();
   }
 

contracts/token/ERC721/ERC721Mintable.sol

@@ -11,6 +11,7 @@ import "../../access/roles/MinterRole.sol";
  */
 contract ERC721Mintable is Initializable, ERC721, MinterRole {
   function initialize() public initializer {
+    ERC721.initialize();
     MinterRole.initialize();
   }