Update tar format to tar-ustar

"tar-ustar selects the ustar format defined by POSIX 1003.1-1988. This format is believed to be old enough to be
portable. It fully supports empty directories. It can store file names with up to 256 characters, provided that the
file name can be split at directory separator in two parts, first of them being at most 155 bytes long. So, in most
cases the maximum file name length will be shorter than 256 characters."

From http://www.gnu.org/software/automake/manual/html_node/List-of-Automake-options.html