package.json: Fix license field.

package.json

@@ -15,11 +15,7 @@
   "bugs": {
     "url": "https://lolg.it/herby/silk/issues"
   },
-  "licenses": [
-    {
-      "type": "MIT"
-    }
-  ],
+  "license": "MIT",
   "engines": {
     "node": "0.10.x || 0.12.x || >=4.0.0"
   },