.travis.yml in test-kitchen-1.20.0 vs .travis.yml in test-kitchen-1.21.0

@@ -3,12 +3,12 @@
 services: docker
 
 language: ruby
 
 rvm:
-  - 2.3
-  - 2.4.2
+  - 2.3.6
+  - 2.4.3
   - ruby-head
 
 
 before_install:
   - gem --version
@@ -24,9 +24,13 @@
   only:
   - master
 
 matrix:
   include:
+    - rvm: 2.5.0
+      before_install:
+      # https://github.com/travis-ci/travis-ci/issues/8978
+      - gem update --system
     - rvm: 2.4
       # To run the proxy tests we need additional gems than what Test Kitchen normally uses
       # for testing
       gemfile: Gemfile.proxy_tests
       before_install: