Add cache_key to BabelProcessor

[rmacklin]

This ensures the cache will be invalidated if BabelProcessor's options are changed. (See this test.)

[rails-bot]

Thanks for the pull request, and welcome! The Rails team is excited to review your changes, and you should hear from @arthurnn (or someone else) soon.

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[rmacklin]

cc @schneems

[schneems]

Thanks!