[BUGFIX lts] Update backburner.js to 2.8.0

Ensure scheduleOnce works correctly following a cancelled job

This fixes the issue described in BackburnerJS/backburner.js#402

(cherry picked from commit bde2e4a)

Author: chancancode

package.json

@@ -77,7 +77,7 @@
     "@simple-dom/interface": "^1.4.0",
     "babel-plugin-debug-macros": "^0.3.4",
     "babel-plugin-filter-imports": "^4.0.0",
-    "backburner.js": "^2.7.0",
+    "backburner.js": "^2.8.0",
     "broccoli-concat": "^4.2.5",
     "broccoli-debug": "^0.6.4",
     "broccoli-file-creator": "^2.1.1",