Enable HTTP/2 support

[weitingchou]

HTTP/2 comes with a lot of new features and enhancements, especially on page load time. Today we have around 75% browsers support the new protocol. It's crucial for our server to support the protocol as well.

see

• https://buzzorange.com/techorange/2016/07/20/http2/
• http://caniuse.com/#search=http2