Update supported Node version to 10.13.0v (#1300)

Author: lahirumaramba

CONTRIBUTING.md

@@ -87,7 +87,7 @@ information on using pull requests.
 
 ### Prerequisites
 
-1. Node.js 10.10.0 or higher.
+1. Node.js 10.13.0 or higher.
 2. NPM 5 or higher (NPM 6 recommended).
 3. Google Cloud SDK ([`gcloud`](https://cloud.google.com/sdk/downloads) utility)
 

README.md

@@ -55,7 +55,7 @@ requests, code review feedback, and also pull requests.
 
 ## Supported Environments
 
-We support Node.js 10.10.0 and higher.
+We support Node.js 10.13.0 and higher.
 
 Please also note that the Admin SDK should only
 be used in server-side/back-end environments controlled by the app developer.