Pull in all of esnext lib (Azure#129)

Fixes Azure#127. I'm not exactly sure why this is necessary, but seeing reports of a similar issue in another lib apollographql/graphql-subscriptions#83

tsconfig.json

@@ -13,7 +13,7 @@
     "target": "es6",
     "sourceMap": true,
     "newLine": "LF",
-    "lib": ["es6", "esnext.asynciterable"]
+    "lib": ["es6", "esnext"]
   },
   "include": ["./src/**/*", "./test/**/*.spec.ts"],
   "exclude": ["node_modules"]