Use GitHub actions for continuous integration (CI)

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

.github/workflows/ci.yml

@@ -0,0 +1,36 @@
+name: CI
+
+on:
+  push:
+  pull_request:
+
+jobs:
+  PHPUnit:
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os:
+          - windows-latest
+          - ubuntu-latest
+        php: 
+          - 5.3
+          - 5.4
+          - 5.5
+          - 5.6
+          - 7.0
+          - 7.1
+          - 7.2
+          - 7.3
+          - 7.4
+    steps:
+      - uses: actions/checkout@v2
+      - name: Setup PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: ${{ matrix.php }}
+          extensions: sockets
+      - 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 }}

README.md

@@ -1,4 +1,6 @@
-# clue/socket-raw [![Build Status](https://travis-ci.org/clue/php-socket-raw.svg?branch=master)](https://travis-ci.org/clue/php-socket-raw)
+# clue/socket-raw
+
+[![CI status](https://github.com/clue/php-socket-raw/workflows/CI/badge.svg)](https://github.com/clue/php-socket-raw/actions)
 
 Simple and lightweight OOP wrapper for PHP's low-level sockets extension (ext-sockets)