fix: assertion failed with elliptic 6.5.4 (#227)

Author: shumkov

package.json

@@ -117,7 +117,7 @@
     "bls-signatures": "^0.2.5",
     "bn.js": "=4.11.8",
     "bs58": "=4.0.1",
-    "elliptic": "^6.5.3",
+    "elliptic": "6.5.3",
     "inherits": "=2.0.1",
     "lodash": "^4.17.20",
     "unorm": "^1.6.0"