Add fastdom js dependency (apache#3947)

The nvd3 docs say that it if the fastdom library is present it makes use
of it.

"Including Fastdom in your project can greatly increase the performance
of the line chart (particularly in Firefox and Internet Explorer) by
batching DOM read and write operations to avoid layout thrashing. NVD3
will take advantage of Fastdom if present."

superset/assets/package.json

@@ -57,6 +57,7 @@
     "datatables.net-bs": "^1.10.15",
     "deck.gl": "^4.1.5",
     "distributions": "^1.0.0",
+    "fastdom": "^1.0.6",
     "geolib": "^2.0.24",
     "immutable": "^3.8.2",
     "jed": "^1.1.1",