Travis: speed up MacPorts using binaries
Travis: speed up MacPorts using binaries

This reduces MacPorts build time by just over 50%. Parent commit build:

  https://travis-ci.org/ocsigen/lwt/builds/138949784

with this commit:

  https://travis-ci.org/ocsigen/lwt/builds/138950982

Related #249.

[skip appveyor]

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