Update config for latest version of Jekyll

Jekyll just had a [nice update](http://jekyllrb.com) that fixes the issue we were having with [timestamps](https://github.com/asyncjs/asyncjs.github.com/commit/8032dbe5603e3f122bb615725bea3462f2e7a940) way back in the day. So I've reverted those changes and updated the `_config.yml` and `Makefile` to use the new timezone feature and stop showing deprecation warnings.