Fix: Spidermonkey dropped --utf8 flag

Four months ago, Spidermonkey dropped the `-U` (`--utf8`) flag (see https://github.com/mozilla/mozilla-central/commit/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).