RxFire: Bump peerDependency of firebase to 6.x.x. (#1944)

* RxFire: Bump peerDependency of firebase to 6.x.x.

* Support both firebase 5 and 6.

Author: yuchenshi

packages/rxfire/package.json

@@ -34,7 +34,7 @@
   "browser": "dist/index.cjs.js",
   "module": "dist/index.esm.js",
   "peerDependencies": {
-    "firebase": "5.x.x",
+    "firebase": ">= 5.0.0 <7",
     "rxjs": "6.x.x"
   },
   "devDependencies": {