Bump gulp-imagemin from 3.3.0 to 4.1.0

Bumps [gulp-imagemin](https://github.com/sindresorhus/gulp-imagemin) from 3.3.0 to 4.1.0.
- [Commits](sindresorhus/gulp-imagemin@v3.3.0...v4.1.0)

package.json

@@ -35,7 +35,7 @@
     "gulp-group-css-media-queries": "^1.2.0",
     "gulp-hub": "github:frankwallis/gulp-hub#registry-init",
     "gulp-if": "^2.0.2",
-    "gulp-imagemin": "^3.3.0",
+    "gulp-imagemin": "^4.1.0",
     "gulp-include": "^2.3.1",
     "gulp-newer": "^1.3.0",
     "gulp-notify": "^3.0.0",