Remove unnecessary sudo setting from travis.yml

[sekky0905]

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What I did

Remove sudo: false setting from travis.yml .

Context

According to the following posts, Travis-CI had two Linux infrastructures which are containers and virtual machines, but recently Travis-CI was combined two Linux infrastructures into a virtual machine-based Linux infrastructures.

https://blog.travis-ci.com/2018-11-19-required-linux-infrastructure-migration https://blog.travis-ci.com/2018-10-04-combining-linux-infrastructures

Now Container-based environment is duplicated.

Build Environment Overview - Travis CI

If you currently specify sudo: false in your .travis.yml, we recommend removing that configuration soon.

quoted by https://blog.travis-ci.com/2018-11-19-required-linux-infrastructure-migration