Remove `ssl_version` option from webhook

Due to the SSLv3 POODLE issue, anyone who has SSLv3 set manually on their webhook won't be able to communicate with servers who (likely) dropped support for SSLv3. This also impacts anyone who uses something like Heroku to host their integration. Given SSLv3 is insecure regardless, I think it's best to completely remove this option.

The documentation also stated that you could set to SSLv2 but that isn't actually true in the code below. This clarifies and relies on TLS.

cc @jdpace @dbussink