Apt-get install should not take tens of minutes to download small packages

Travis is taking 3min 25s to download a 19.1 MB package at 282 kB/s. This is absurd. I shouldn’t have to wait nearly 10 minutes just for the dependencies to install. See, e.g., https://travis-ci.com/github/JasonGross/fiat/jobs/316704760, but also every other build in this repository running at around the same time.