fix(@angular/cli): add a dependency to RXJS (#4465)

Projects dont have a problem with it because they have that dependency directly, but
global installs dont have rxjs and will error out.

Author: hansl

packages/@angular/cli/lib/cli/index.js

@@ -1,8 +1,9 @@
 /*eslint-disable no-console */
 
-// Prevent the dependency validation from tripping because we don't import zone.js. We need
+// Prevent the dependency validation from tripping because we don't import these. We need
 // it as a peer dependency of @angular/core.
 // require('zone.js')
+// require('rxjs')
 
 
 // This file hooks up on require calls to transpile TypeScript.