Travis: eliminate some redundant build rows
Travis: eliminate some redundant build rows

Also test on 4.04 on macOS Homebrew.

[skip appveyor]

rgrinberg/lwt,c-cube/lwt,rneswold/lwt,ocsigen/lwt,c-cube/lwt,rgrinberg/lwt,ocsigen/lwt,rgrinberg/lwt,c-cube/lwt,ocsigen/lwt,rneswold/lwt,rgrinberg/lwt,rneswold/lwt