Four months ago, Spidermonkey dropped the `-U` (`--utf8`) flag (see mozilla/mozilla-central@53c97c#L16L4876). That change breaks (*) amo-validator when the latest version of Spidermonkey is installed. This proposal fixes the bug while maintaining backwards-compatible.
If you want to completely drop support for old Spidermonkey versions, removing `'-U'` from line 170 also fixes the bug.
(*) "breaks" = amo-validator crashes with a simplejson.decoder.JSONDecodeError, because `data` is not valid JSON. In fact, `data` holds the same value as `js --help` (Spidermonkey usage).