please allow ltsp 5.1.5-1 into testing

[vagrant@freegeek.org]

ltsp is again blocked from migrating into testing, likely due to the
ltsp-client-builder udeb, though this udeb is not used by
debian-installer by default.

it has been in unstable for 8 days now, without introducing new
problems.

thanks for your consideration and work.

live well,
  vagrant