update swift package deps, fix #221

Author: skywinder

Package.swift

@@ -10,7 +10,7 @@ let package = Package(
     .library(name: "web3swift", targets: ["web3swift"]),
     ],
   dependencies: [
-    .package(url: "https://github.com/attaswift/BigInt.git", from: "3.1.0"),
+    .package(url: "https://github.com/attaswift/BigInt.git", from: "5.0"),
     .package(url: "https://github.com/mxcl/PromiseKit.git", from: "6.8.4"),
     .package(url: "https://github.com/daltoniam/Starscream.git", from: "3.1.0"),
     .package(url: "https://github.com/krzyzanowskim/CryptoSwift.git", from: "1.0.0"),