Pin fast-async at v6.3.1 to fix installation

package.json

@@ -22,7 +22,7 @@
     "ethereumjs-util": "^5.1.2",
     "execr": "^1.0.1",
     "exorcist": "^0.4.0",
-    "fast-async": "^6.1.2",
+    "fast-async": "6.3.1",
     "fast-levenshtein": "^2.0.6",
     "javascript-serialize": "^1.6.1",
     "jquery": "^3.3.1",