mailcatcher: fix missing assets

[sj26]

Without this step the user interface does not function because the javascript and stylesheets are not compiled or included, so cannot be served to browsers.

This command was run in the formula for v0.9.0 but was removed in the update to v0.10.0 (#173471):

https://github.com/Homebrew/homebrew-core/commit/104b175d45a9454103e5a25fd2ff6f8749eeeea6#diff-aff4c5be3f339d85e769e3c2362bb626acfde080d57d0bb75f42551ecc3d0413L161

Closes https://github.com/sj26/mailcatcher/issues/560.

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• [x] Have you built your formula locally with HOMEBREW_NO_INSTALL_FROM_API=1 brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
• [x] Is your test running fine brew test <formula>, where <formula> is the name of the formula you're submitting?
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