fix: upgrade pngjs from 2.2.0 to 2.3.1

Snyk has created this PR to upgrade pngjs from 2.2.0 to 2.3.1.

See this package in npm:
https://www.npmjs.com/package/pngjs

See this project in Snyk:
https://app.snyk.io/org/dceejay/project/d423d0d8-60aa-4eae-823a-1cbe4be8a581?utm_source=github&utm_medium=referral&page=upgrade-pr

hardware/unicorn/package.json

@@ -3,7 +3,7 @@
     "version"       : "0.1.1",
     "description"   : "A Node-RED node to output to a Raspberry Pi Unicorn HAT from Pimorini.",
     "dependencies"  : {
-        "pngjs": "2.2.*"
+        "pngjs": "2.3.1"
     },
     "repository" : {
         "type":"git",