Test multisite against PHP 7.3 w/ 5.2 and PHP 5.6 with 5.1

Author: Shawn Hooper

.travis.yml

@@ -24,10 +24,12 @@ matrix:
     - php: 5.5
       env: WP_VERSION=5.1
       dist: precise
+    - php: 5.6
+      env: WP_VERSION=5.1 WP_MULTISITE=1
     - php: 7.3
-      env: WP_VERSION=latest WP_MULTISITE=1
+      env: WP_VERSION=5.2 WP_MULTISITE=1
     - php: 7.3
-      env: WP_VERSION=4.5 WP_MULTISITE=1
+      env: WP_VERSION=latest WP_MULTISITE=1
     - php: 7.3
       env: NPM_TESTS=1