Change Debian package to build against libdb4.8

  libdb4.7 isn't in stable (lenny) and libdb4.8 is in testing (squeeze)
  and sid, so given that we can do the auto upgrade no reason not to
  bump to use the latest. Tested with 20000+ invocations of "onak index"
  to try to confirm no repeat of the read-only-open problem hit with 4.6.