MB-9089: force update of favicon

To reflect latest logo change. Which apparently doesn't always work
automagically due to aggressive caching of favicons in browsers

Change-Id: I90aff1bcfe7a4956bed7564569374108ff62b665
Reviewed-on: http://review.couchbase.org/28864
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>