chore(ci): use github action instead of travis-ci (#99)

Author: Slashgear

.github/workflows/node.js.yml

@@ -0,0 +1,26 @@
+name: Continous Integration
+
+on:
+  push:
+    branches: [master]
+  pull_request:
+    branches: [master]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        node-version: [8.x, 10.x, 12.x, 14.x, 15.x]
+        superagent-version: [3.6.0,3.7.0,3.8.0,4.1.0]
+
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: Use Node.js ${{ matrix.node-version }}
+        uses: actions/setup-node@v1
+        with:
+          node-version: ${{ matrix.node-version }}
+
+      - run: yarn install
+      - run: yarn add superagent@~${{ matrix.superagent-version }} && yarn ci

.travis.yml

@@ -1,7 +1,10 @@
-![](https://img.shields.io/badge/License-MIT-00CCFF.svg?style=flat-square)
-![](https://img.shields.io/badge/superagent--mock-JS-FF0066.svg?style=flat-square)
-[![NPM Downloads](http://img.shields.io/npm/dm/superagent-mock.svg?style=flat-square)](https://www.npmjs.org/package/superagent-mock)
-[![Build Status](http://img.shields.io/travis/M6Web/superagent-mock.svg?style=flat-square)](https://travis-ci.org/M6Web/superagent-mock)
+![npm](https://img.shields.io/npm/v/superagent-mock)
+![npm bundle size](https://img.shields.io/bundlephobia/minzip/superagent-mock)
+![npm](https://img.shields.io/npm/dy/superagent-mock)
+![GitHub last commit](https://img.shields.io/github/last-commit/M6web/superagent-mock)
+![NPM](https://img.shields.io/npm/l/superagent-mock)
+[![NPM Downloads](http://img.shields.io/npm/dm/superagent-mock.svg)](https://www.npmjs.org/package/superagent-mock)
+[![Continous Integration](https://github.com/M6Web/superagent-mock/actions/workflows/node.js.yml/badge.svg)](https://github.com/M6Web/superagent-mock/actions/workflows/node.js.yml)
 
 <p align="center">
 <b><a href="#installation">Installation</a></b>
@@ -10,8 +13,6 @@
 |
 <b><a href="#supported-methods">Supported Methods</a></b>
 |
-<b><a href="#tests">Tests</a></b>
-|
 <b><a href="#credits">Credits</a></b>
 |
 <b><a href="#license">License</a></b>
@@ -31,7 +32,7 @@ Install with [yarn](https://yarnpkg.com/): `yarn add superagent-mock`
 
 ## Requirements
 
-node >= 6
+node >= 8.0
 superagent >= ^3.6.0
 
 ## Usage