Avoid rubygems 3.x that doesn't support ruby < 2.3

see rubygems/rubygems#2534

.travis.yml

@@ -13,8 +13,8 @@ gemfile:
   - gemfiles/5.1.gemfile
   - gemfiles/5.2.gemfile
 before_install:
+  - gem i rubygems-update -v '<3' && update_rubygems
   - gem install bundler
-  - gem update --system
   - gem --version
   - curl -L https://codeclimate.com/downloads/test-reporter/test-reporter-latest-linux-amd64 > ./cc-test-reporter
   - chmod +x ./cc-test-reporter