fix(perf): remove async dependency

Author: mzgoddard

package.json

@@ -21,7 +21,6 @@
   },
   "dependencies": {
     "ajv": "6.3.0",
-    "async": "2.6.0",
     "gzip-js": "0.3.2",
     "jszip": "3.1.5",
     "pify": "4.0.1"