Do not rely on web3 injection with new ethereum provider standard

when window.ethereum is available web3 is likely not available as the
browser uses the new provider standard.
So we either user the window.ethereum as provider or the
web3.currentProvider as legacy option.

app/services/kredits.js

@@ -50,9 +50,9 @@ export default Service.extend({
         });
       }
 
-      function instantiateWithAccount (web3, context) {
+      function instantiateWithAccount (web3Provider, context) {
         console.debug('[kredits] Using user-provided instance, e.g. from Mist browser or Metamask');
-        ethProvider = new ethers.providers.Web3Provider(web3.currentProvider);
+        ethProvider = new ethers.providers.Web3Provider(web3Provider);
         ethProvider.listAccounts().then(accounts => {
           context.set('currentUserAccounts', accounts);
           const ethSigner = accounts.length === 0 ? null : ethProvider.getSigner();
@@ -64,19 +64,18 @@ export default Service.extend({
       }
 
       if (window.ethereum) {
-        window.web3 = new window.Web3(window.ethereum);
         try {
           // Request account access if needed
           await window.ethereum.enable();
           // Acccounts now exposed
-          instantiateWithAccount(window.web3, this);
+          instantiateWithAccount(window.ethereum, this);
         } catch (error) {
           instantiateWithoutAccount();
         }
       }
       // Legacy dapp browsers...
       else if (window.web3) {
-        instantiateWithAccount(window.web3, this);
+        instantiateWithAccount(window.web3.currentProvider, this);
       }
       // Non-dapp browsers...
       else {