.travis.yml in fastly-2.2.0 vs .travis.yml in fastly-2.3.0

@@ -1,13 +1,13 @@
 language: ruby
 script: 'bundle exec rake test:unit'
 sudo: false
 matrix:
   include:
-    - rvm: 2.1.10
+    - rvm: 2.3.8
       gemfile: gemfiles/HEAD.gemfile
-    - rvm: 2.2.7
+    - rvm: 2.4.5
       gemfile: gemfiles/HEAD.gemfile
-    - rvm: 2.3.4
+    - rvm: 2.5.3
       gemfile: gemfiles/HEAD.gemfile
-    - rvm: 2.4.1
+    - rvm: 2.6.0
       gemfile: gemfiles/HEAD.gemfile