allow use without solidus_backend

[ccarruitero]

Summary

Currently solidus_subscriptions has references to solidus_backend, even when solidus_backend isn't a dependency on gemspec.

It will be easier just add solidus_backend as dependency in gemspec, but I think the plan with solidus_admin is to eventually replace solidus_backend. So, this PR allows use solidus_subscriptions extension without solidus_backend.

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