.travis.yml in fluent-plugin-out-http-1.1.3 vs .travis.yml in fluent-plugin-out-http-1.1.4

@@ -1,19 +1,23 @@
 rvm:
-  - 2.1
   - 2.2
   - 2.3
-  - 2.4.0
+  - 2.4.5
+  - 2.5.3
+  - 2.6.0
   - ruby-head
 
 os:
   - linux
   - osx
 
 dist: trusty
 
 gemfile:
   - Gemfile
+
+before_install:
+  - gem update --system=2.7.8
 
 script: bundle exec rake test
 
 matrix:
   allow_failures: