CI: Add 2.6

  - also - Remove an old setting from Travis. See https://blog.travis-ci.com/2018-11-19-required-linux-infrastructure-migration

.travis.yml

@@ -1,7 +1,6 @@
 ---
 language: ruby
 dist: trusty
-sudo: false
 cache: bundler
 script: "bundle exec rake test"
 rvm:
@@ -31,6 +30,9 @@ matrix:
     - rvm: 2.5
       before_install:
         - gem install bundler
+    - rvm: 2.6
+      before_install:
+        - gem install bundler
     - rvm: ruby-head
       before_install:
           - gem install bundler