Travis: remove 4.06 allow_failures rows
Travis: remove 4.06 allow_failures rows

Lwt, all its dependencies, and all helper libraries now build on 4.06.

[skip appveyor]

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