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.

Test Plan:
Toggled showGutter on and off and saw gutter change width
oops, forgot to fix lint errors after recording gif :)
Merge branch 'master' into polite-gutter

Conflicts:
	lib/gutter-view.coffee
	lib/linter-view.coffee
	stylesheets/linter.less
Merge pull request #137 from dmnd/polite-gutter

Only widen the gutter when showGutter is enabled