Use SVG images for tour

SVG images will look sharper and scale more cleanly. However they are
not supported on older browsers[1]

> The <picture> element allows for fallback images when the browser
> doesn't support a specified image format.

`<picture>` is supported in the latest versions of Chrome, Firefox, IE
(Edge), Safari and Android, so all these browsers will get the SVG
version.

Older browsers will fall back to the PNG version.

[1] http://caniuse.com/#feat=svg
[2] https://css-tricks.com/a-complete-guide-to-svg-fallbacks/#fallback-svg-as-img-picture
[3] http://caniuse.com/#feat=SVG

app/assets/stylesheets/components/banner.scss

@@ -131,7 +131,8 @@
 
   }
 
-  img {
+  img,
+  source {
     max-width: 100%;
     display: block;
   }