Add expected matrix for .travis.yml

Add PHP version 7.3, 7.4 into matrix build

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

Author: yukihiko-shinoda

features/scaffold-plugin-tests.feature

@@ -61,6 +61,10 @@ Feature: Scaffold plugin unit tests
       """
       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

features/scaffold-theme-tests.feature

@@ -59,6 +59,10 @@ Feature: Scaffold theme unit tests
       """
       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

features/scaffold.feature

@@ -478,6 +478,10 @@ Feature: WordPress code scaffolding
       """
       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