package.json licence field update to GPL-3.0+

Apparently the SPDX license list used by NPM supports an "or later"
identifier; that's good, as this is an incredibly important distinction; I
was otherwise going to drop it and use my own custom identifier.

* package.json: License field set to GPL-3.0{=>+}

package.json.in

@@ -13,7 +13,7 @@
   "dependencies": {},
   "devDependencies": {},
 
-  "license": "GPL-3.0",
+  "license": "GPL-3.0+",
 
   "scripts": {
     "test": "make test"