Bump optipng-bin from 6.0.0 to 9.0.0 (closes #176)

Bumps [optipng-bin](https://github.com/imagemin/optipng-bin) from 6.0.0 to 9.0.0.
- [Release notes](https://github.com/imagemin/optipng-bin/releases)
- [Commits](https://github.com/imagemin/optipng-bin/compare/v6.0.0...v9.0.0)

---
updated-dependencies:
- dependency-name: optipng-bin
  dependency-type: direct:development
  update-type: version-update:semver-major
...

Change-Id: I9c93ecdea17eead6f0291b18ed039db0635eb14a
Signed-off-by: dependabot[bot] <support@github.com>
diff --git a/package.json b/package.json
index bfe28de..bacf810 100644
--- a/package.json
+++ b/package.json
@@ -20,7 +20,7 @@
     "grunt-shell": "^4.0.0",
     "grunt-terser": "^1.0.0",
     "imagemin": "^8.0.1",
-    "optipng-bin": "^6.0.0",
+    "optipng-bin": "^9.0.0",
     "sass": "^1.44.0",
     "vinyl-fs": "^3.0.3"
   },