fix: Ensure the load order of the optional dependency of Rackup

[NikolayRys]

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Description of changes

rails_semantic logger has an optional dependency on Rackup.

Unfortunately, Bundler doesn't have a concept of an optional dependency, meaning that it doesn't enforce the loading of gems that aren't declared as a requirement in the gemspec. This means that rails_semantic_logger behavior actually depends on the ordering inside the Gemfile, and I don't think that was anyone's intention.

This change force-preloads Rackup if it's available or works a usual if not.

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[rgaufman]

Please merge

[FraDim]

@reidmorrison I think this pull request will resolve

• #211

[NikolayRys]

It will indeed, should have noted it in the description 👍