Only widen the gutter when showGutter is enabled

After installing linter the gutter becomes wider. Even when the config option
showGutter is disabled the gutter is still wider than it should be. This change
only adds gutter CSS when showGutter is enabled.

![06urt1m93v](https://cloud.githubusercontent.com/assets/4427/3592484/ac88d3ba-0c72-11e4-8654-6e50b13ba1df.gif)