.travis.yml: The 'sudo' tag is now deprecated in Travis CI (…

[Travis are now recommending removing the __sudo__ tag](https://blog.travis-ci.com/2018-11-19-required-linux-infrastructure-migration).

"_If you currently specify __sudo: false__ in your __.travis.yml__, we recommend removing that configuration_"

cclauss authored Dominic Jodoin committed

Nothing here #10515.1 passed

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