Merge pull request #608 from devopsdays/greenkeeper/gulp-imagemin-4.0.0

Update gulp-imagemin to the latest version 🚀

Author: mattstratton

package.json

@@ -23,7 +23,7 @@
     "gulp-cache": "^1.0.0",
     "gulp-concat": "^2.6.1",
     "gulp-htmlmin": "^3.0.0",
-    "gulp-imagemin": "^3.1.1",
+    "gulp-imagemin": "^4.0.0",
     "gulp-img-retina": "0.0.4",
     "gulp-rename": "^1.2.2",
     "gulp-responsive": "^2.7.0",