Remove `sudo: false` from `.travis.yml`

Since this repository was not recognized by Travis before 2015-01-01,
there is no need to include `sudo: false` as that is now the default.

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From https://docs.travis-ci.com/user/workers/container-based-infrastructure/#Routing-your-build-to-container-based-infrastructure:

 " The default behavior, when no sudo usage is detected in
   any customizable build phases, depends on the date when
   the repository is first recognized by Travis CI:

   * For repos we recognize before 2015-01-01, linux builds
     are sent to our standard infrastructure.

   * For repos we recognize on or after 2015-01-01, linux
     builds are sent to our container-based infrastructure "