Add cache buster to production builds

[julianlam]

Running into issues where static assets are cached by the user (intended, of course), but where we'd ideally like to have assets expired the moment NodeBB is updated.

So if caching is on (NODE_ENV is not development), add a cache buster equal to the timestamp retrieved via the git log.

git log --pretty=format:'%at' -n 1

http://try.nodebb.org/css/theme.css?v=1385679062

[cnvo]

I've been having problems with this issue as well at times and would often need to clear my cache. I figured it was just me but I guess not, so I'm glad this will be implemented. 👍