Add missing @validatem/core devDependency

example.js

@@ -1,7 +1,7 @@
 "use strict";
 
 const required = require("./");
-const { validateValue } = require("@validatem/core"); /* FIXME: Add to devDeps */
+const { validateValue } = require("@validatem/core");
 
 console.log(validateValue(42, [ required ])); // 42
 

package.json

@@ -5,5 +5,8 @@
   "main": "index.js",
   "repository": "http://git.cryto.net/validatem/required.git",
   "author": "Sven Slootweg <admin@cryto.net>",
-  "license": "WTFPL OR CC0-1.0"
+  "license": "WTFPL OR CC0-1.0",
+  "devDependencies": {
+    "@validatem/core": "^0.3.1"
+  }
 }