chore(package): update broccoli-concat to version 4.2.3

Closes #80

Author: greenkeeper[bot]

package.json

@@ -12,7 +12,7 @@
   },
   "devDependencies": {
     "@types/qunit": "^2.0.31",
-    "broccoli-concat": "^3.2.2",
+    "broccoli-concat": "^4.2.3",
     "broccoli-debug": "^0.6.4",
     "broccoli-funnel": "^2.0.1",
     "broccoli-merge-trees": "^3.0.0",