title: Cannot find module "rxjs/observable" on build. IONIC
body: <p>I am working on an ionic app that is using AngularFire2. But when I push up the code to ionic i get a build error:</p>
  
  <pre><code> ionic-app-scripts build
  
  
  [22:28:07]  typescript: src/providers/sermon/sermon.ts, line: 4 
              Cannot find module 'rxjs/observable'. 
  
         L3:  import { AngularFirestore, AngularFirestoreCollection} from 'angularfire2/firestore';
         L4:  import { Observable } from 'rxjs/observable';
  
  [22:28:07]  ionic-app-script task: "build" 
  [22:28:07]  Error: Failed to transpile program 
  Error: Failed to transpile program
  </code></pre>
  
  <p>I don't know what is going on... Could someone point me in the right direction for this?</p>