Update GitHub Actions

Author: plegner

.github/workflows/test.yml

@@ -0,0 +1,17 @@
+name: CI Tests
+on:
+  pull_request:
+  push:
+    branches:
+      - master
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions/setup-node@v2
+        with:
+          node-version: '12'
+      - run: npm ci
+      - run: npm test
+      - run: npm run lint

.travis.yml

@@ -1,6 +1,6 @@
 # Fermat.ts
 
-[![Build Status](https://travis-ci.org/mathigon/fermat.js.svg?branch=master)](https://travis-ci.org/mathigon/fermat.js)
+[![Build Status](https://github.com/mathigon/fermat.js/workflows/CI%20Tests/badge.svg)](https://github.com/mathigon/fermat.js/actions?query=workflow%3A%22CI+Tests%22)
 [![npm](https://img.shields.io/npm/v/@mathigon/fermat.svg)](https://www.npmjs.com/package/@mathigon/fermat)
 [![npm](https://img.shields.io/github/license/mathigon/fermat.js.svg)](https://github.com/mathigon/fermat.js/blob/master/LICENSE)