Update zen-observable-ts to eliminate transitive dependency on @types/zen-observable

[benjamn]

This update includes apollographql/zen-observable-ts#152, and should fix issue #8688 because Apollo Client no longer generates code in dist/**/*.d.ts files with type references to the @types/zen-observable package:

/// <reference types="zen-observable" />

With these references gone, consumers of zen-observable-ts should not need to add @types/zen-observable as a dependency. In fact, the @types/zen-observable package is no longer involved at all. Thus, #8688 should be fixed.

Since this could be a disruptive type-level change for anyone who was previously depending on the global declaration of the ZenObservable namespace, I'm targeting the release-3.5 branch so we can test before releasing.

[brainkim]

LGTM