Update fetch signature

[nicksieger]

Fixes the following error related to the generated formula in recent Homebrew installations:

Error: gem-tmuxinator: Calling `def fetch` in a subclass of `Formulary::FormulaNamespace7d1fdbf89fe8034169754cfa3d37ea30::RubyGemsDownloadStrategy` is disabled! Use `def fetch(timeout: nil, **options)` and output a warning when `options` contains new unhandled options instead.

[production-status-check]

Has QA approval