currencyrate: switch to new coindesk api

[daywalker90]

The old coindesk api is no longer working. I'm not sure this is the best index but the values seem in line with the other sources.

[chrisguida]

This appears to fix the CI failure here on line 1560:

pyln.client.lightning.RpcError: RPC call failed: method: currencyrates, payload: ['USD'], error: {'code': -32600, 'message': 'Error while processing currencyrates: HTTPSConnectionPool(host=\'api.coindesk.com\', port=443): Max retries exceeded with url: /v1/bpi/currentprice/USD.json (Caused by NameResolutionError("<urllib3.connection.HTTPSConnection object at 0x7f9012cdaf00>: Failed to resolve \'api.coindesk.com\' ([Errno -5] No address associated with hostname)"))', .

ACK 51c2046

Thanks for the fix @daywalker90 !!