Cannot run https

### Expected behaviour
Should allow https
### Actual behaviour
Chrome returns ERR_SSL_VERSION_OR_CIPHER_MISMATCH and firefox a similar version.
### Environment Details

- Karma version (output of `karma --version`): 2.0.0

- Relevant part of your `karma.config.js` file
```
httpsServerConfig: {
key: fs.readFileSync('./tests/assets/privateKey.key', 'utf8'),
cert: fs.readFileSync('./tests/assets/certificate.crt', 'utf8')
},
protocol: 'https:',
```
### Steps to reproduce the behaviour

1. write a test that needs to be run in HTTPS
2. generate certificates
3. run karma.

### Other infos ###
[x] I didn't set hostname or proxy.
[x] I've also used the certs that could be found in this repo (test directory)