Support nodejs version 12.x at least

Author: WhatAKitty

.travis.yml

@@ -1,6 +1,4 @@
 language: node_js
 node_js:
-  - '6.17.1'
-  - '7.8.0'
   - '12.19.0'
   - '15.0.1'

README.md

@@ -12,6 +12,10 @@ fetch mock for react-native
 No fetch mock could be used easily for react-native.  
 So, I create one by myself.
 
+## Requirements
+
+Nodejs >= 12.x
+
 ## Roadmap
 - [x] Combined with Mock.js
 - [x] Support exclude for some other path