Update firebase dependencies and fix breaking changes in lib

CHANGELOG.MD

@@ -1,4 +1,7 @@
 # Changelog
+## 5.1.0
+* Add support for firebase@^8.0.0
+
 ## 5.0.1
 * Add support for uiShow Callback [#133](https://github.com/RaphaelJenni/FirebaseUI-Angular/issues/133)
 

README.MD

@@ -30,22 +30,13 @@ import {BrowserModule} from '@angular/platform-browser';
 import {NgModule} from '@angular/core';
 import {FormsModule} from '@angular/forms';
 import {AppComponent} from './app.component';
+import {firebase, firebaseui, FirebaseUIModule} from 'firebaseui-angular';
 import {environment} from '../environments/environment';
 import {AppRoutingModule} from './app-routing.module';
-
-
-import {FirebaseUIModule} from 'firebaseui-angular';
-import * as firebase from 'firebase/app';
-import * as firebaseui from 'firebaseui';
-// currently there is a bug while building the app with --prod
-// - https://github.com/RaphaelJenni/FirebaseUI-Angular/issues/76
-// the plugin exposes the two libraries as well. You can use those:
-import {FirebaseUIModule, firebase, firebaseui} from 'firebaseui-angular';
-
-
 import {AngularFireModule} from '@angular/fire';
 import {AngularFireAuthModule} from '@angular/fire/auth';
 
+
 const firebaseUiAuthConfig: firebaseui.auth.Config = {
   signInFlow: 'popup',
   signInOptions: [
@@ -95,6 +86,8 @@ export class AppModule {
 }
 
 
+
+
 ```
 
 ### Add the firebaseui css to your imports:
@@ -221,7 +214,7 @@ uiShownCallback() {
 }
 ```
 
-### Disable autosign
+### Disable auto sign-in
 
 ```typescript
 constructor(private firebaseuiAngularLibraryService: FirebaseuiAngularLibraryService) {