Switch config upgrade to use xdr instead of json

[leighmcculloch]

The quickstart image needs changing how it does config upgrades, so instead of giving stellar-core a json file for the config to upgrade to, it gives it xdr.

In #513 we added config upgrades using the json format.

However, stellar-core is being updated to remove the json dependency and will just accept xdr.

The stellar-core command we started using in 513 already supports both xdr and json, so we can switch at any point but need to switch before the v20 release of stellar-core.

cc @sisuresh @anupsdf