fix rubygems version to test ruby 2.1 and 2.2

repeatedly · repeatedly · commit ed7c675ec135 · 2018-12-19T19:59:24.000+09:00
diff --git a/.travis.yml b/.travis.yml
@@ -13,7 +13,8 @@ services:
   - mongodb
 
 before_install:
-  - gem update --system
+  - gem update --system=2.7.8
+
 before_script:
   - git submodule update -i
 
