travis: cleanups and simplify our use of the apt addon

travis: cleanups and simplify our use of the apt addon

- Remove some extraneous parameters to the apt addon since it does not
  seem to start since I switched to bionic. The doc says that some
  features were removed in that addon, but I'm not sure of whether it
  should affect us given our config. Let's see if it's better this way.
- Kill dead code about lua. 5.3 is part of Debian since a very long time
- Reindent the build rules for linux and osx (no change)