Properly identify POSIX uncompressed tars

[RuiPereira]

The first 6 bytes are not enough to identify POSIX
uncompressed tars (257 bytes offset), so changed
the magic_bytes to read the first 262 bytes.

This is almost the same as #10226. Can be tested with a POSIX tar with this (dummy) formula:

require 'formula'

class SrbClient < Formula
  homepage 'http://www.sdsc.edu/srb/index.php'
  url 'http://www.sdsc.edu/srb/tarfiles/SRB3_4_2client.tar'
  md5 'cb42947abbabd7677ef4c960fc2ce2be'
  version '3.4.2'

  def install
    system "./configure", "--disable-debug", "--disable-dependency-tracking"
  end
end