update rxjs to be compatible with rxjs 6

[aciccarello]

The rxjs 6 release candidate is out now with several breaking changes. When users update to rxjs 6, they can continue to use ng-metadata in its current state by adding the rxjs-compat library, however, npm will warn about version compatibility. ng-metadata should be updated to rxjs 6 to allow users to use rxjs 6 in their applications without rxjs-compat.

If ng-metadata won't be update to rxjs 6, it should still be updated to use pipeable operators

• Use only pipeable operators
• import all RxJS symbols from top level imports only

See the RxJS v5.5-v6 update guide for details.