v2.21.0

Highlights

• Rails. 6.x compatibility
• Improved payload truncation
• Improved handling of circular references in payload
• Removed dependency on MultiJson

Pull requests

• Enable Rails 6.x and fix RollbarTest load dependency issue, #882
• Use dup to avoid FrozenError, #885
• Enable last resort truncation strategy: delete (nearly) any key, #888
• Use ruby json.generate to serialize payloads, #890
• Expect to see and handle non-json objects, #891