[#26] Use SSL_CTX_use_certificate_chain_file

(reference https://github.com/criticalstack/libevhtp/issues/26)

Via @mannol

```
Currently, libevhtp is using SSL_CTX_use_certificate_file to load a
certificate file. That function lacks the ability to load the pinned
certificate chain (if any) which has a consequence of connecting clients
not trusting the received certificate. By using
SSL_CTX_use_certificate_chain_file we give the libssl the ability to
read and send the entire certificate chain (if any), which clients can
check against.
```
1 file changed