Fix CI builds for Lwt contributors

[aantron]

Some contributors have set up CI testing in their own GH forks of the repo, e.g. in #367 and #347. It seems that their builds don't work until they add sudo: required to their .travis.yml, e.g. d198a2b.

I am not sure why the main repo's builds are working without that line. Perhaps they are getting grandfathered in after some breaking change in Travis, which the Lwt build predates. I guess we should just add sudo: required to the main .travis.yml, and make sure everything is working.

This issue is basically about making that change, opening a PR against the main repo, and waiting to make sure the builds don't break. To avoid triggering the AppVeyor builds, one can add [skip appveyor] to the commit message.

I want to note for contributors, that it's okay to submit a PR to the main repo, and just use it for triggering CI builds while still developing the change – just add a note in the PR that it's not ready for review. However, we shouldn't annoy people who choose to set up their own CI builds for whatever reason.

EDIT: cc @jsthomas @domsj

[persianturtle]

I'll take care of this one.