Added temporary dependency on typedoc fork.

Due to an issue[1] in typedoc's mixin support,
the documentation generation phase of the
release process fails.

This commit points the typedoc dependency to a
custom fork[2] that contains a fix. We can and
should back this change out later when
mainline is fixed.

[1] TypeStrong/typedoc#1207
[2] https://github.com/zslayton/typedoc/

package.json

@@ -76,7 +76,7 @@
     "source-map-support": "^0.5.13",
     "ts-node": "^8.4.1",
     "tslint": "^5.20.1",
-    "typedoc": "^0.15.0",
+    "typedoc": "github:zslayton/typedoc#7f58d79b0a445434e487d6b61bb14933fbe0c329",
     "typescript": "^3.6.2"
   },
   "dependencies": {