.travis.yml in sidekiq-5.2.4 vs .travis.yml in sidekiq-5.2.5

@@ -2,10 +2,10 @@
 sudo: false
 cache: bundler
 services:
   - redis-server
 before_install:
-  - gem update --system
+  - gem install bundler -v '< 2'
 gemfile:
   - gemfiles/rails_4.gemfile
   - gemfiles/rails_5.gemfile
 bundler_args: --without development load_test
 rvm: