Switch from travis-ci to github actions

.. because my travis "balance is negative" and I do not plan to give them money.

.github/workflows/test.yml

@@ -0,0 +1,46 @@
+name: JsonMapper tests
+on: [push, workflow_dispatch]
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        php-versions:
+          - '7.1'
+          - '7.2'
+          - '7.3'
+          - '7.4'
+          - '8.0'
+          - '8.1'
+          - '8.2'
+    name: PHP ${{ matrix.php-versions }}
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v3
+
+      - name: Install PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: ${{ matrix.php-versions }}
+          tools: composer
+          coverage: xdebug
+
+      - name: Get composer cache directory
+        id: composer-cache
+        run: echo "dir=$(composer config cache-files-dir)" >> $GITHUB_OUTPUT
+
+      - name: Cache dependencies
+        uses: actions/cache@v3
+        with:
+          path: ${{ steps.composer-cache.outputs.dir }}
+          key: ${{ runner.os }}-composer-${{ matrix.php-versions }}-${{ hashFiles('**/composer.lock') }}
+          restore-keys: ${{ runner.os }}-composer-${{ matrix.php-versions }}-
+
+      - name: Install dependencies
+        run: composer install --no-interaction --prefer-dist
+
+      - name: Run unit tests
+        run: XDEBUG_MODE=coverage ./vendor/bin/phpunit --coverage-text
+
+      - name: Check codestyle
+        run: ./vendor/bin/phpcs --standard=PEAR src/