fix(package): update broccoli-plugin to version 3.1.0

Closes #1

Author: greenkeeper[bot]

package.json

@@ -18,7 +18,7 @@
   ],
   "dependencies": {
     "broccoli-merge-trees": "^3.0.1",
-    "broccoli-plugin": "^2.1.0"
+    "broccoli-plugin": "^3.1.0"
   },
   "ember-addon": {
     "configPath": "tests/dummy/config"