Fastlane seems to fail with some json gem conflict

On https://circleci.com/gh/mediapop/patontheback-mobile/320

fastlane beta --verbose
/usr/local/lib/ruby/site_ruby/2.3.0/rubygems/specification.rb:2278:in `check_version_conflict': can't activate json-2.0.2, already activated json-1.8.3 (Gem::LoadError)

The solution is to use bundler instead of relying on the system dependencies: https://docs.fastlane.tools/best-practices/ContinuousIntegration/#circle-integration