.travis.yml: no additional apt packages for spec/ruby [ci skip]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65147 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

.travis.yml

@@ -78,6 +78,9 @@ matrix:
       language: ruby
       rvm: 2.3
       name: Running ruby/spec on 2.3 to ensure version guards are correctly added
+      addons:
+        apt:
+          packages:
       before_install:
       install:
       before_script: chmod -R u+w spec/ruby