fix(libcurl_http_fetcher): use a standard ssl certificate location

Before the update_engine only looked at our self-hosted copy of the
update service. But, other update service's can exist. Let them use a
regular SSL certificate and put it into /etc/certs/ssl.
Merge pull request #27 from philips/new-update-url

fix(libcurl_http_fetcher): use a standard ssl certificate location