.travis.yml in fluent-plugin-mongo-1.2.1 vs .travis.yml in fluent-plugin-mongo-1.2.2

@@ -11,10 +11,11 @@
 
 services:
   - mongodb
 
 before_install:
-  - gem update --system
+  - gem update --system=2.7.8
+
 before_script:
   - git submodule update -i
 
 script: bundle exec rake test