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/benchpress/package.json

@@ -7,7 +7,7 @@
   "dependencies": {
     "@angular/core": "^2.0.0-rc.7",
     "reflect-metadata": "^0.1.2",
-    "rxjs": "5.0.0-beta.12",
+    "rxjs": "5.0.0-rc.4",
     "jpm": "1.1.4",
     "firefox-profile": "0.4.0",
     "selenium-webdriver": "^2.53.3"

modules/@angular/core/package.json

@@ -8,7 +8,7 @@
   "author": "angular",
   "license": "MIT",
   "peerDependencies": {
-    "rxjs": "5.0.0-beta.12",
+    "rxjs": "5.0.0-rc.4",
     "zone.js": "^0.6.21"
   },
   "repository": {

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"
   },

modules/@angular/router/package.json

@@ -24,6 +24,6 @@
     "@angular/core": "0.0.0-PLACEHOLDER",
     "@angular/common": "0.0.0-PLACEHOLDER",
     "@angular/platform-browser": "0.0.0-PLACEHOLDER",
-    "rxjs": "5.0.0-beta.12"
+    "rxjs": "5.0.0-rc.4"
   }
 }