build: update dgeni packages (angular#12956)

* Upgrades to the latest Dgeni packages version that includes a fix which ensures that the docs for the `Platform` show up properly.

See: angular/dgeni-packages@ab957d1

Author: devversion

package-lock.json

@@ -73,7 +73,7 @@
     "chalk": "^1.1.3",
     "codelyzer": "^4.3.0",
     "dgeni": "^0.4.10",
-    "dgeni-packages": "^0.26.7",
+    "dgeni-packages": "^0.26.9",
     "firebase": "^4.0.0",
     "firebase-admin": "^5.0.0",
     "firebase-tools": "^4.1.0",