[observable] Upgrade rxjs to 5.5.6, adding .pipe operator

[bjoerge]

RxJS 5.5 introduced the concept of "lettable operators", which is a great way to reduce bundle size by importing just the operators needed, e.g.:

import {map, filter, scan} from 'rxjs/operators';

//...

client.observable
  .listen('*[_id == "somedoc"')
  .pipe(
    filter(doc => doc.isPublished),
    map(doc => doc.count),
    scan((acc, count) => acc + count, 0)
  )
  .subscribe(totalCount => console.log(totalCount))

[rexxars]

LGTM!