Add PHP version 7.3, 7.4 into matrix on Travis CI

Since current PHP recommended version is 7.3+.
@see https://wordpress.org/support/article/requirements/

Author: yukihiko-shinoda

templates/plugin-travis.mustache

@@ -18,6 +18,10 @@ cache:
 
 matrix:
   include:
+    - php: 7.4
+      env: WP_VERSION=latest
+    - php: 7.3
+      env: WP_VERSION=latest
     - php: 7.2
       env: WP_VERSION=latest
     - php: 7.1