.travis.yml: revert "workaround to resolve ::1"

This commit reverts r60736.

Shibata-san reported the cause of r60736 to Travis and it seems fixed:
https://github.com/travis-ci/travis-ci/issues/8780

In another CI, I found "::1     ip6-localhost ip6-loopback" in build
system information, unlike "::1  localhost ip6-localhost ip6-loopback"
that had failed in Ruby's CI on Travis.

Let's try Container-based environment again.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@60886 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

.travis.yml

@@ -19,7 +19,7 @@
 language: c
 dist: trusty
 
-sudo: required
+sudo: false
 
 compiler:
   - gcc