:bug: bugfix for Bundler 1.16.1 and rubygems 2.7.3 on Travis
:bug: bugfix for Bundler 1.16.1 and rubygems 2.7.3 on Travis

When we use Bundler 1.16.1 and rubygems 2.7.3 on Travis environment, test will fail.
So we need upgrade.

https://github.com/rubygems/rubygems/issues/2123

resque/redis-namespace