Bump postcss from 8.2.15 to 8.3.0 (#65)

Author: dependabot[bot]

package-lock.json

@@ -19,7 +19,7 @@
     "gulp-svgmin": "^3.0.0",
     "gulp-uglify": "^3.0.0",
     "plugin-error": "^1.0.1",
-    "postcss": "^8.2.15",
+    "postcss": "^8.3.0",
     "request": "^2.79.0",
     "request-promise-native": "^1.0.3"
   },