Consider making Makefile Debian-aware

[ltalirz]

The Makefile currently only works on Debian, if one follows the instructions in the README and runs apt-get install socat first.

Otherwise, in trying to install socat, the Makefile will try to add the universe repository here: https://github.com/dokku/dokku-daemon/blob/c36a2460f9f7737bb4fb6621f6961773933a1409/Makefile#L38 which is specific to Ubuntu and does not exist for Debian (Debian has main, contrib and non-free).

As suggested by @josegonzalez , the Makefile could check

OS=$(shell lsb_release -si)

And then do a conditional if that is Ubuntu or Debian.

[josegonzalez]

Just seeing this!

As always, Pull requests welcome :)