[BUGFIX beta] Update Backburner.js to 2.5.0.

The primary change here is to use the Promise microtask queue primarily
and falling back to MutationObserver when Promise is not present.

package.json

@@ -108,7 +108,7 @@
     "babel-plugin-filter-imports": "^2.0.4",
     "babel-plugin-module-resolver": "^3.2.0",
     "babel-template": "^6.26.0",
-    "backburner.js": "^2.4.2",
+    "backburner.js": "^2.5.0",
     "broccoli-babel-transpiler": "^7.1.2",
     "broccoli-concat": "^3.7.3",
     "broccoli-debug": "^0.6.4",

yarn.lock

@@ -1356,10 +1356,10 @@ backbone@^1.1.2:
   dependencies:
     underscore ">=1.8.3"
 
-backburner.js@^2.4.2:
-  version "2.4.2"
-  resolved "https://registry.yarnpkg.com/backburner.js/-/backburner.js-2.4.2.tgz#4cc2d3e78592ddd46ae9a48cfc168ca883b56e6b"
-  integrity sha512-YFgfwWvelbJ9JxsVF+sph7nLB2qQfwDyUXCr+ZPyeU6HOXGAT0Rt+ro+mU1zRMnUfYG57nwaTiz1yqvqNO6jQQ==
+backburner.js@^2.5.0:
+  version "2.5.0"
+  resolved "https://registry.yarnpkg.com/backburner.js/-/backburner.js-2.5.0.tgz#d03ab8f7b77e708ac4bc7bea2258aa57177b560d"
+  integrity sha512-YkLvnh0jEA67NNJ+NqJrX4Yb/isanR7XHlb+Y+cMXkGtXzlgx2tHEnZPIWi2kbW9EwtCq3HTRcAF+mbzIhfoqg==
 
 backo2@1.0.2:
   version "1.0.2"