feat: update RxJS peer dependency to 5.0.0-rc.4

Closes #13125

RxJS from beta-12 to rc.4, has removed the `cache`
operator. (See https://github.com/ReactiveX/rxjs/blob/master/CHANGELOG.md#breaking-changes-1)
If your application relies on it, then we suggest 
that you use the one from this gist:
https://gist.github.com/robwormald/19dea0c70a6e01aadced6731aed4f9f7

modules/@angular/http/package.json

@@ -8,7 +8,7 @@
   "author": "angular",
   "license": "MIT",
   "peerDependencies": {
-    "rxjs": "5.0.0-beta.12",
+    "rxjs": "5.0.0-rc.4",
     "@angular/core": "0.0.0-PLACEHOLDER",
     "@angular/platform-browser": "0.0.0-PLACEHOLDER"
   },