Use GitHub actions for continuous integration (CI)

Bye bye Travis CI, you've served us well.

Author: clue

.github/workflows/ci.yml

@@ -0,0 +1,31 @@
+name: CI
+
+on:
+  push:
+  pull_request:
+
+jobs:
+  PHPUnit:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        php:
+          - 7.4
+          - 7.3
+          - 7.2
+          - 7.1
+          - 7.0
+          - 5.6
+          - 5.5
+          - 5.4
+    steps:
+      - uses: actions/checkout@v2
+      - name: Setup PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: ${{ matrix.php }}
+      - run: composer install
+      - run: vendor/bin/phpunit --coverage-text
+        if: matrix.php >= 7.3
+      - run: vendor/bin/phpunit --coverage-text -c phpunit.xml.legacy
+        if: matrix.php < 7.3

.travis.yml

@@ -1,4 +1,6 @@
-# clue/reactphp-eventsource [![Build Status](https://travis-ci.org/clue/reactphp-eventsource.svg?branch=master)](https://travis-ci.org/clue/reactphp-eventsource)
+# clue/reactphp-eventsource
+
+[![CI status](https://github.com/clue/reactphp-eventsource/workflows/CI/badge.svg)](https://github.com/clue/reactphp-eventsource/actions)
 
 Event-driven EventSource client, receiving streaming messages from any HTML5 Server-Sent Events (SSE) server,
 built on top of [ReactPHP](https://reactphp.org/).