Set ServerName in default.conf, not apache.conf

We write a line to the end of the apache.conf setting the servername explicitly - this probably makes more sense to be done in default.conf, where we configure the kbhff site. We also don't need to do this during build, because the hostname of the apache server is always going to be `kbhff.local` once we've fixed #9 (and should be `apache` prior to that I think).