Update ethers.js to latest version v4

The main change is how ethers loads the networkId which is now async.
Thus the init process had to change a bit

lib/contracts/base.js

@@ -17,11 +17,7 @@ class Base {
   }
 
   on(type, callback) {
-    let eventMethod = `on${type.toLowerCase()}`;
-    // Don't use this.contract.events here. Seems to be a bug in ethers.js
-    this.contract[eventMethod] = callback;
-
-    return this;
+    return this.contract.on(type, callback);
   }
 }
 module.exports = Base;

lib/contracts/kernel.js

@@ -12,7 +12,9 @@ class Kernel extends Base {
   }
 
   appNamehash(appName) {
-    return AppIds[this.contract.provider.chainId.toString()][appName];
+    // actually provider.network is an asynchronous property.
+    // but when we call this function kredits is already initialized and the network is already loaded
+    return AppIds[this.contract.provider.network.chainId.toString()][appName];
   }
 }