add rxjs as core package peerDependency (#76)

Already used, just wasn't specified

Author: bengry

libs/core/package.json

@@ -54,6 +54,7 @@
     "@angular/core": "^7.0.3",
     "@angular/platform-browser-dynamic": "^7.0.3",
     "@angular/platform-browser": "^7.0.3",
+    "rxjs": "^6.2.0",
     "react-dom": "^16.6.3",
     "react": "^16.6.3"
   },