Use of `serialized_attributes` is deprecated and will be removed in Rails 5.0

Our codebase migrated to Rails 4.2 recently and we are seeing a lot of deprecation warnings. One of them appears to be getting fired from Identity Cache:

DEPRECATION WARNING: `serialized_attributes` is deprecated without replacement, and will be removed in Rails 5.0

Relevant Rails PR: rails/rails#15704

[dylanahsmith]

Fixed by pull request #208