Remove deprecated `sudo: false` in .travis.yml

	- https://blog.travis-ci.com/2018-11-19-required-linux-infrastructure-migration
	- https://docs.travis-ci.com/user/reference/overview/#deprecated-virtualization-environments

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_6@67178 b2dd03c8-39d4-4d8f-98ff-823fe69b080e