Dock icon not hidden on OS X

[jonmmease]

As discussed in #66 (comment), in order to hide the electron icon from popping up in the dock on OS X the "extendInfo": {"LSUIElement": 1} attribute should be added to the project's package.json.

I thought this had been done, and I remember testing that it worked (successfully hid the icon), but I don't see this option present on master, and the icon is not hidden when I use the dmg install on OS X.

Here's the commit that added the config d185fcf. I'm guessing it just got lost in the shuffle when PR #66 was closed in favor of #72