Attempt to guard against google-analytics failure on Download link

If Google-Analytics doesn't call the hitCallback, the Download link will
do nothing, which I guess is what happened here:

https://twitter.com/tamaslnagy/status/425104242794188800

Workaround taken from:

https://www.domsammut.com/code/workaround-for-when-the-hitcallback-function-does-not-receive-a-response-analytics-js

In a quick experiment using FireFox and AdBlockPlus v2.4.1, I wasn't able
to reproduce the issue, but there seems no harm in adding this guard
(possibly tamaslnagy has a different ad-blocker?).