Use GitHub actions for continuous integration (CI)

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

.gitattributes

@@ -1,6 +1,6 @@
 /.gitattributes export-ignore
+/.github/ export-ignore
 /.gitignore export-ignore
-/.travis.yml export-ignore
 /phpunit.xml.dist export-ignore
 /phpunit.xml.legacy export-ignore
-/tests export-ignore
+/tests/ export-ignore

.github/workflows/ci.yml

@@ -0,0 +1,28 @@
+name: CI
+
+on:
+  push:
+  pull_request:
+
+jobs:
+  PHPUnit:
+    name: PHPUnit (PHP ${{ matrix.php }})
+    runs-on: ubuntu-20.04
+    strategy:
+      matrix:
+        php:
+          - 7.4
+          - 7.3
+          - 7.2
+          - 7.1
+    steps:
+      - uses: actions/checkout@v2
+      - uses: shivammathur/setup-php@v2
+        with:
+          php-version: ${{ matrix.php }}
+          coverage: xdebug
+      - 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 }}

.gitignore

@@ -1,4 +1,2 @@
-composer.lock
-composer.phar
-build/
-vendor/
+/composer.lock
+/vendor/

README.md

@@ -4,8 +4,7 @@ Promise
 A lightweight implementation of
 [CommonJS Promises/A](http://wiki.commonjs.org/wiki/Promises/A) for PHP.
 
-[![Build Status](https://travis-ci.org/reactphp/promise.svg?branch=master)](http://travis-ci.org/reactphp/promise)
-[![Coverage Status](https://coveralls.io/repos/github/reactphp/promise/badge.svg?branch=master)](https://coveralls.io/github/reactphp/promise?branch=master)
+[![CI status](https://github.com/reactphp/promise/workflows/CI/badge.svg)](https://github.com/reactphp/promise/actions)
 
 > The master branch contains the code for the upcoming 3.0 release.
 > For the code of the current stable 2.x release, checkout the