Use Rails.configuration.identity_cache_store to set cache backend

`config.identity_cache_store` is other then in the [README](https://github.com/Shopify/identity_cache/blob/master/README.md#installation) defined not used.

This pull request, implements the behaviour defined in the [README](https://github.com/Shopify/identity_cache/blob/master/README.md#installation).

See also issue #27