libcurl_http_fetcher: disable SSLv3 for HTTPS connections.

SSLv3 is broken and should not be used. Ideally we should restrict this
further and only use TLSv1.2 but that may be a problem with some obscure
corporate proxy setups. Possibly implement that later with a
configuration option but for now allow all TLSv1.x versions.
Merge pull request #52 from marineam/tls

libcurl_http_fetcher: disable SSLv3 for HTTPS connections.