adding properly scoped dependency for core

Author: dmolsen

composer.json

@@ -25,7 +25,7 @@
   },
   "require": {
     "php":              ">=5.4",
-    "pattern-lab/core": "dev-dev",
+    "pattern-lab/core": "^2.4.0",
     "wrench/wrench":    "^2.0.0",
     "cocur/background-process": "~0.5"
   },