Updated instructions for Debian (and derivates)

A more up-to-date instruction to install osmose-backend
osmosis in stable is 0.40.1 but in testing/unstable we have 0.43.1 (so it should be fine with a testing/sid install)
osmctools is only available in testing/unstable; osmconvert is 0.7T

For Ubuntu it's similar: osmosis 0.43.1 only in utopic and osmctools since trusty.