Remove co usage in favor of async/await.

Author: Robert Jackson

package.json

@@ -49,7 +49,6 @@
   },
   "devDependencies": {
     "broccoli-test-helper": "^2.0.0",
-    "co": "^4.6.0",
     "coveralls": "^3.0.3",
     "eslint": "^5.14.0",
     "eslint-config-prettier": "^4.0.0",