Do not upgrade Homebrew
Do not upgrade Homebrew

I believe we don't need the upgrade, so skipping it will save us some
time (19 minutes with `upgrade`[1] vs. 13 minutes without it[2]).

1. https://travis-ci.org/newsboat/newsboat/jobs/450294148
2. https://travis-ci.org/newsboat/newsboat/jobs/441293061

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