Warn about srcset not being used

lib/metadata/policies.js

@@ -846,7 +846,7 @@ var policies = {
     "totalWeight": {
         "tool": "weightChecker",
         "label": "Total weight",
-        "message": "<p>The weight is of course very important if you want the page to load fast. Try to stay under 1MB, which is alreay very long to download over a slow connection.</p><p>Please note that Yellow Lab Tools' engine (PhantomJS) is not compatible with image srcset, unless you use a polyfill. This can lead to incorrect page weight.</p>",
+        "message": "<p>The weight is of course very important if you want the page to load fast. Try to stay under 1MB, which is alreay very long to download over a slow connection.</p><p>Please note that Yellow Lab Tools' engine (PhantomJS) is not compatible with image srcset (unless you use a polyfill). This can lead to incorrect page weight.</p>",
         "isOkThreshold": 716800,
         "isBadThreshold": 2097152,
         "isAbnormalThreshold": 3145728,
@@ -856,7 +856,7 @@ var policies = {
     "imageOptimization": {
         "tool": "weightChecker",
         "label": "Image optimization",
-        "message": "<p>This metric mesures the number of bytes that could be saved by optimizing images.</p><p>Image optimization is generally one of the easiest way to reduce a page weight, and as a result, the page load time. Don't use Photoshop or other image editing tools, they're not very good for optimization. Use specialized tools such as <a href=\"https://kraken.io/\" target=\"_blank\">Kraken.io</a> or the excellent <a href=\"https://imageoptim.com/\" target=\"_blank\">ImageOptim</a> on Mac. For SVG images, you can use <a href=\"https://jakearchibald.github.io/svgomg/\" target=\"_blank\">SVGOMG</a></p><p>The tools in use in YellowLabTools are not set to their maximum optimization power (JPEG quality 85), so you might be able to compress even more!</p><p>Please note that Yellow Lab Tools' engine (PhantomJS) is not compatible with image srcset, unless you use a polyfill. This can lead to incorrect page weight.</p>",
+        "message": "<p>This metric mesures the number of bytes that could be saved by optimizing images.</p><p>Image optimization is generally one of the easiest way to reduce a page weight, and as a result, the page load time. Don't use Photoshop or other image editing tools, they're not very good for optimization. Use specialized tools such as <a href=\"https://kraken.io/\" target=\"_blank\">Kraken.io</a> or the excellent <a href=\"https://imageoptim.com/\" target=\"_blank\">ImageOptim</a> on Mac. For SVG images, you can use <a href=\"https://jakearchibald.github.io/svgomg/\" target=\"_blank\">SVGOMG</a></p><p>The tools in use in YellowLabTools are not set to their maximum optimization power (JPEG quality 85), so you might be able to compress even more!</p><p>Please note that Yellow Lab Tools' engine (PhantomJS) is not compatible with image srcset (unless you use a polyfill). This can lead to incorrect page weight.</p>",
         "isOkThreshold": 10240,
         "isBadThreshold": 122880,
         "isAbnormalThreshold": 307200,