To solve this problem I downgraded to the previous version (v.14) of the geckobinary. v.15 seems to be incompatible with the rest of my environment (Ubuntu 14.04, Firefox 52.0.1).

Script above now outputs:

Binary mode: 1
Google
[download]

Note however, that Selenium::Firefox does not successfully terminate the Firefox process with shutdown_binary() (only the geckodriver process), so each time you call new() you will be left with a running Firefox instance until you kill it manually. This problem appears to be similar to one discussed here.