Do not break custom pricing options classes

[mamhoff]

Summary

Prior to this commit, installing this gem would break custom pricing options classes in host apps: By overriding the getter on Spree::Config, it was very difficult to use a custom pricing options class. By instead decorating the base class, we make sure that even if host app developers have used their own pricing options class, that pricing options class gets the right cache_key method (provided they themselves overrode cache_key using super).

Checklist

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[kennyadsl]

Thanks Martin!