fix(timer): setInterval should not auto cancel after callback invoked, fix rxjs version to pass CI

[JiaLiPassion]

fix #934.
setInterval will auto clear timerId after first time invoked.
the issue was imported several versions ago, but it doesn't become a problem because we
return ZoneTask from setInterval before zone.js 0.8.17, from zone.js 0.8.18 , setInterval will return the original timerId, so when we call clearInterval the timerId have been cleared, which will cause zone.js onHasTask not be called, the setInterval task was always there.
And will cause ngZone not stable.

Also fix rxjs version to 5.4.2 to avoid karam error, I will continue to figure out how to resolve karam test error with rxjs 5.5.0.
The issue is here, and it seems it will be fixed in rxjs 5.5.1
ReactiveX/rxjs#2971

[adrienverge]

Hi @JiaLiPassion,

I have tested this PR and I confirm it solves the setInterval() / clearInterval() problem (issues #934 and #945)!

Thanks a lot 👍

Do we need to wait for rxjs to publish a new version to merge this, or not?

[JiaLiPassion]

@adrienverge, thank you for confirm, we don't need to wait for rxjs new version, because zone.js doesn't depends on rxjs, but I am not sure when this will be merged.

[sarunint]

Since rxjs issue mentioned is resolved, shouldn't you change the rxjs version to ^5.4.2 again?

[JiaLiPassion]

@sarunint , yes, I will update the package.json.