.travis.yml in appium_lib-9.17.0 vs .travis.yml in appium_lib-9.18.0

@@ -1,15 +1,15 @@
 sudo: false
 cache: bundler
 language: ruby
 rvm:
-  - 2.2.5
-  - 2.3.3
-  - 2.4.0
-  - 2.5.0
+  - 2.2
+  - 2.3
+  - 2.4
+  - 2.5
+  - 2.6
 
 before_install:
-  - gem update --system
   - gem update bundler
 install:
   - bundle install --retry=3
 script:
   - bundle exec rake rubocop