build: fix dashboard deployment (#9118)

Fixes the dashboard failing to deploy due to an error in `firebase@4.8.1`. Locks down the version to 4.8.0 for now. See angular/angularfire#1385.

Author: crisbeto

tools/dashboard/package.json

@@ -24,7 +24,7 @@
     "angularfire2": "^5.0.0-rc.2",
     "core-js": "^2.4.1",
     "d3": "^4.9.1",
-    "firebase": "^4.5.0",
+    "firebase": "4.8.0",
     "rxjs": "^5.4.2",
     "zone.js": "^0.8.12"
   },