1. SPDY has been deprecated and removed from modern browsers:
http://caniuse.com/#feat=spdy
2. HTTP/2 is present in all modern browsers:
http://caniuse.com/#feat=http2
3. SPDY and HTTP/2 are mutually exclusive in port-based `listen` directives:
http://nginx.org/en/docs/http/ngx_http_core_module.html#listen
Caniuse suggests that 74% of global users have a browser that supports
HTTP/2, while only 27% have a browser that supports SPDY. Only IE11 and
Safari support both.
Therefore, when both are available, Dokku should choose HTTP/2 to
maximize the benefit to users.
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