Add fallback rate backend providers

[kincaidoneil]

When CoinCap goes down, everything stops working...which is bad.

[tmlee]

May i suggest considering the CoinGecko open public API https://www.coingecko.com/api ?
APIs are quite easy to consume to get what you need, here is an example https://api.coingecko.com/api/v3/simple/price?ids=bitcoin,ethereum,ripple&vs_currencies=usd,eur,gbp
Happy to answer any questions or concerns you may have!