Use WP 5.0-beta5 when running PHP 7.3 in Travis

To make it easier to test WC running the upcoming PHP 7.3, this commit changes the PHP 7.3 Travis build to run unit tests using WP 5.0-beta5. This way we won't display WP core warnings related to PHP 7.3 that were already fixed in the upcoming WP version and will be able to see only WC related warnings.

My initial plan was to use WP nightly but that didn't work as nightly is still build from trunk and WP 5.0 development is being done on another branch (https://make.wordpress.org/core/2018/10/05/wordpress-5-0-commit-management/).

.travis.yml

@@ -13,7 +13,6 @@ php:
   - 5.6
   - 7.0
   - 7.1
-  - 7.3
 
 env:
   - WP_VERSION=latest WP_MULTISITE=0
@@ -31,9 +30,13 @@ matrix:
     env: WP_VERSION=latest WP_MULTISITE=0 RUN_PHPCS=1 RUN_E2E=1
   - php: 7.1
     env: WP_VERSION=latest WP_MULTISITE=0 RUN_CODE_COVERAGE=1
-  allow_failures:
-  - env: WP_VERSION=latest WP_MULTISITE=0 RUN_CODE_COVERAGE=1
   - php: 7.3
+    env: WP_VERSION=5.0-beta5 WP_MULTISITE=0
+  allow_failures:
+  - php: 7.1
+    env: WP_VERSION=latest WP_MULTISITE=0 RUN_CODE_COVERAGE=1
+  - php: 7.3
+    env: WP_VERSION=5.0-beta5 WP_MULTISITE=0
 
 before_script:
   - |