Downgrade ember-concurrency to 4.x to keep support for @task

Author: cyril-sf

package.json

@@ -39,7 +39,7 @@
     "ember-auto-import": "^2.11.1",
     "ember-cli-babel": "^8.2.0",
     "ember-cli-htmlbars": "^6.3.0",
-    "ember-concurrency": "^5.1.0",
+    "ember-concurrency": "^4.0.6",
     "ember-on-resize-modifier": "^2.0.2",
     "ember-ref-bucket": "^5.0.8",
     "ember-set-helper": "^3.1.0",