Bump pngjs from 5.0.0 to 6.0.0

Bumps [pngjs](https://github.com/lukeapage/pngjs) from 5.0.0 to 6.0.0.
- [Release notes](https://github.com/lukeapage/pngjs/releases)
- [Commits](https://github.com/lukeapage/pngjs/commits)

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

package.json

@@ -96,7 +96,7 @@
     "mock-socket": "^9.0.3",
     "nightwatch": "^1.4.3",
     "open": "^7.3.0",
-    "pngjs": "^5.0.0",
+    "pngjs": "^6.0.0",
     "pre-commit": "^1.2.2",
     "request": "^2.88.0",
     "selenium-server": "3.141.59",