Add a Travis CI build using the minimum WooCommerce requirements

The current build matrix is always testing against the latest version of WordPress, but WooCommerce officially supports WordPress 5.0 and newer.

.travis.yml

@@ -50,6 +50,9 @@ matrix:
   - name: "WooCommerce unit tests using WordPress nightly"
     php: 7.4
     env: WP_VERSION=nightly WP_MULTISITE=0
+  - name: "Minimum requirements"
+    php: 5.6
+    env: WP_VERSION=5.0 WP_MULTISITE=0
   allow_failures:
   - php: 7.4
     env: WP_VERSION=latest WP_MULTISITE=0 RUN_CODE_COVERAGE=1