OpenSSL::SSL::SSLErrors in test suite

CI on `master` is presently broken, with SSL-related tests failing with the following error:

https://travis-ci.org/github/httprb/http/jobs/744656845

```
#<OpenSSL::SSL::SSLError: SSL_connect returned=1 errno=0 state=error: sslv3 alert unsupported certificate>
```