Improve aesthetics of Safari toolbar icon

The new icon I made looks like this on a Retina display:

![retina display screenshot](http://i.imgur.com/qXrkaHN.png)

And like this on a non-Retina display:

![1x display screenshot](http://i.imgur.com/1jaeeni.png)

The old icon had no 2x version and didn't really look like the other Safari toolbar items:

![old icon screenshot](http://i.imgur.com/AqYB1P5.png)