.travis.yml: try `addons.apt.update: true` for gcc-8

because gcc-8 setup is super unstable like this
https://travis-ci.org/ruby/ruby/jobs/441528813

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65083 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

.travis.yml

@@ -29,6 +29,7 @@ matrix:
       compiler: gcc-8
       addons:
         apt:
+          update: true
           sources:
             - ubuntu-toolchain-r-test
           packages: