Update readme example to use 0.5.0.

README.md

@@ -17,7 +17,7 @@ npm install openzeppelin-solidity
 To write your custom contracts, import ours and extend them through inheritance.
 
 ```solidity
-pragma solidity ^0.4.24;
+pragma solidity ^0.5.0;
 
 import 'openzeppelin-solidity/contracts/token/ERC721/ERC721Full.sol';
 import 'openzeppelin-solidity/contracts/token/ERC721/ERC721Mintable.sol';