Update default cipher since stream cipher support was discontinued

[Snawoot]

This PR updates default cipher because latest release of shadowsocks clients discontinues support for stream ciphers. This way everything will still work by default with latest clients.

chacha20-ietf-poly1305 cipher was suggested as default because of following reasons:

• This cipher has great performance both on mobile devices without AES-NI and desktops. Won't hurt much.
• It's a good secure AEAD cipher proposed by independent researchers and accepted as a modern standard.