Add PHP 7.4 to the list of Travis build jobs

This commit adds the upcoming PHP 7.4 to the list of Travis build jobs
so that we can start testing WooCommerce with this new version of PHP
that is scheduled to be released in November.

.travis.yml

@@ -46,9 +46,13 @@ matrix:
   - name: "WooCommerce unit tests using WordPress nightly"
     php: 7.3
     env: WP_VERSION=nightly WP_MULTISITE=0
+  - php: 7.4snapshot
+    env: WP_VERSION=nightly WP_MULTISITE=0
   allow_failures:
   - php: 7.3
     env: WP_VERSION=latest WP_MULTISITE=0 RUN_CODE_COVERAGE=1
+  - php: 7.4snapshot
+    env: WP_VERSION=nightly WP_MULTISITE=0
 
 before_script:
   - export PATH="$HOME/.composer/vendor/bin:$PATH"