build: update to rxjs 6.4

Author: Alan Agius

package.json

@@ -23,7 +23,7 @@
   "dependencies": {
     "@angular-devkit/core": "0.0.0",
     "enhanced-resolve": "4.1.0",
-    "rxjs": "6.3.3",
+    "rxjs": "6.4.0",
     "tree-kill": "1.2.1",
     "webpack-sources": "1.3.0"
   },