Remove sudo:false in .travis.yml

[y-yu]

• Travis CI requires to remove sudo:false option to move into Linux infrastructure. See also:
  • Upcoming Required Linux Infrastructure Migration
• The official document says that Container-based infrastructure has been fully deprecated
  • https://docs.travis-ci.com/user/reference/trusty/#container-based-infrastructure
• So I dropped sudo: false
• I think that other repositories in this organization(for example https://github.com/ocaml/dune) are also using sudo:false, it would be better to do the same fix. May I submit the same PR to other https://github.com/ocaml repositories?

[dra27]

Thanks for the change - without lessening it, we don't normally do Changes entries for CI updates - did you only push that second commit to satisfy the Travis test?

[dra27]

Thanks!

[y-yu]

@dra27

Thank you.

we don't normally do Changes entries for CI updates

Sorry 🙇 I just thought all CI tests should pass so I was planning to remove the Changes commit (a53c051) if such a changing wouldn't be written in Changes.... So I don't mind if you will remove my entry in Changes.