[BUGFIX release] Update backburner.js to 2.4.0.

* Fixes an issue that occurs when things like `lolex` /
`sinon.useFakeTimers()` / etc.
* Add new `getDebugInfo` API to expose timers, counters, scheduled
tasks, etc.

package.json

@@ -93,7 +93,7 @@
     "babel-plugin-transform-es2015-template-literals": "^6.22.0",
     "babel-plugin-transform-object-assign": "^6.22.0",
     "babel-template": "^6.26.0",
-    "backburner.js": "^2.3.0",
+    "backburner.js": "^2.4.0",
     "broccoli-babel-transpiler": "^6.1.4",
     "broccoli-concat": "^3.2.2",
     "broccoli-debug": "^0.6.4",

yarn.lock

@@ -1169,10 +1169,10 @@ backbone@^1.1.2:
   dependencies:
     underscore ">=1.8.3"
 
-backburner.js@^2.3.0:
-  version "2.3.0"
-  resolved "https://registry.yarnpkg.com/backburner.js/-/backburner.js-2.3.0.tgz#074c1c277c5c6f1c7c0087e3b617f92129eef562"
-  integrity sha512-9WfbgbedI3HHQEaPA6fwy/9YFvCwtRKW95pzirfrMJ4WtIdHQmerGAmHgUzQGRdnVKorIun1aIoL3JxzNV6GgQ==
+backburner.js@^2.4.0:
+  version "2.4.0"
+  resolved "https://registry.yarnpkg.com/backburner.js/-/backburner.js-2.4.0.tgz#77252984c5ac0139103023fadc1085b768e8155f"
+  integrity sha512-B9f7QmTOO1ZN8IL0d39Gm4xOL3tg/RumYC+jMSNQDAsxvfHaQZEwI5teFUyZuPptGNdeCtT5z6TsPXp2eWy2og==
 
 backo2@1.0.2:
   version "1.0.2"