xar: Use github.tarball_from archive

[ryandesign (Ryan Carsten Schmidt)]

The xar port uses the github portgroup and does not specify a value for github.tarball_from, therefore it uses the default value, tarball, which is deprecated. When you update the port to the next version, please add the line github.tarball_from archive to the Portfile. (Adding this line before updating to the next version will cause checksum mismatches.)