The Coin Market Cap API is now deprecated and the new version does not offer a public API.

Deprecation warning: https://api.coinmarketcap.com/
Migration information: https://pro.coinmarketcap.com/migrate

When this takes effect the CoinMarketCap class (https://github.com/decred/decred-php-api/blob/master/src/Decred/Rate/CoinMarketCap.php) will stop working.