Bump CI things (#768)

* Update rubygems in a simpler way

* Use latest rubygems

* Use the latest patch level rubies

* Use whatever rubygems comes with ruby

.travis.yml

@@ -1,16 +1,15 @@
 language: ruby
 
 before_install:
-  - gem i rubygems-update -v '3.0.4' && update_rubygems
   - gem i bundler -v '2.0.2'
 
 bundler_args: --without benchmark --jobs=3 --retry=3
 
 matrix:
   include:
-    - rvm: 2.4.6
-    - rvm: 2.5.5
-    - rvm: 2.6.3
+    - rvm: 2.4.9
+    - rvm: 2.5.7
+    - rvm: 2.6.5
     - rvm: jruby-9.2.9.0
       env: JRUBY_OPTS=--debug
       jdk: openjdk8