Upgrade impro to its latest version 0.13.1.

This includes a patch for an apparent issue with a non-Error being
emitted on an inner stream. Note that this moves sharp up to 0.30.0
and some test files that change as a result are regenerated.

package.json

@@ -16,7 +16,7 @@
     "gm-papandreou": "^1.23.0-patch1",
     "hijackresponse": "^5.0.0",
     "httperrors": "^2.0.1",
-    "impro": "~0.11.0",
+    "impro": "~0.13.1",
     "inkscape": "^3.0.0",
     "jpegtran": "^2.0.0",
     "mime": "^3.0.0",
@@ -25,7 +25,7 @@
     "passerror": "^1.1.1",
     "pngcrush": "^3.0.0",
     "pngquant": "^4.0.0",
-    "sharp": "^0.28.2"
+    "sharp": "^0.30.0"
   },
   "devDependencies": {
     "browser-sync": "^2.18.6",