Replace template’s `j1` with `deparallelize`.

If you guys would actually accept a PR that un-defines `ENV.j1` and replaces every existing usage of it with `ENV.deparallelize`, I'll close this PR and open a new one that subsumes this patch.

Anyway, the one commit's message follows:

Solely because I think `ENV.deparallelize` is *way* clearer than
`ENV.j1`, at least to the uninitiated.

Also, updating the template will reduce the number of `ENV.j1`s that
persist because the Homebrew maintainers want clean histories of
formulae. Ideally, this change should prevent any more cases from being
introduced!