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 their servers.
This especially 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.

cc @jdpace @dbussink
Merge pull request #949 from github/remove-ssl-option-from-web

Remove `ssl_version` option from webhook