mediawiki.hidpi: Add skipFunction to forgo loading in modern browsers

mediawiki.hidpi: Add skipFunction to forgo loading in modern browsers

Chromium 34 (stable release April 8, 2014) introduced support
for the srcset attribute.

Chroem 34+ and Opera 21+ support this now. While the jquery.hidpi
module doesn't emulate srcset in browser that support it, we do
still load it.

http://blog.chromium.org/2014/02/chrome-34-responsive-images-and_9316.html
http://googlechromereleases.blogspot.com/2014/04/stable-channel-update.html
http://caniuse.com/srcset

Change-Id: I0498c9cc0f25e3d366879d9d3430ee8371efcf11