chore(package): update gulp-imagemin to version 5.0.0

Author: greenkeeper[bot]

package.json

@@ -22,7 +22,7 @@
     "gulp-concat": "^2.6.1",
     "gulp-htmlmin": "^5.0.1",
     "gulp-hub": "^4.2.0",
-    "gulp-imagemin": "^4.0.0",
+    "gulp-imagemin": "^5.0.0",
     "gulp-img-retina": "0.0.4",
     "gulp-rename": "^1.2.2",
     "gulp-responsive": "^2.11.0",