http2bin.org is down

Looks like the example code no longer works since the sever http2bin.org is down: 

```
from hyper import HTTPConnection

conn = HTTPConnection('http2bin.org:443')
conn.request('GET', '/get')
resp = conn.get_response()

print(resp.read())
````

<https://github.com/kennethreitz/httpbin/issues/309#issuecomment-339558664>