Update license declaration in package.json (#77)

The LICENSE file was changed to GPLv2 in #14, but this was not reflected
in package.json.

Author: binaryf0x

package.json

@@ -8,7 +8,7 @@
     "url": "git+https://github.com/Cellivar/WebZLP.git"
   },
   "author": "Cellivar Kangaroorat <[email protected]> (https://github.com/cellivar)",
-  "license": "GPL-3.0-only",
+  "license": "GPL-2.0",
   "bugs": {
     "url": "https://github.com/Cellivar/WebZLP/issues"
   },