Set login shell to brew's zsh when available.
Set login shell to brew's zsh when available.

* https://github.com/thoughtbot/laptop/commit/401107bcdddb8758f95630453b5d59b5e3af54a2

joshukraine/mac-bootstrap,joshukraine/mac-bootstrap,sjdweb/mac-bootstrap,sjdweb/mac-bootstrap,joshukraine/mac-bootstrap,tadelv/mac-bootstrap,tadelv/mac-bootstrap,tadelv/mac-bootstrap