Fix display for canvas

If not set to "block", we get a 5px gap at the bottom.
Ref https://stackoverflow.com/questions/15807833/div-containing-canvas-have-got-a-strange-bottom-margin-of-5px

Author: vidartf

js/src/_base/Renderable.js

@@ -423,7 +423,7 @@ class RenderableView extends widgets.DOMWidgetView {
         this.$renderer = $(this.renderer.domElement);
         this.$el.empty().append(this.$renderer);
 
-        this.$el.css('margin-bottom', '-5px');
+        this.$el.css('display', 'block');
 
         this.updateSize();