確保 travis 在 php 8.x 能裝到 phpunit 7

疑似關聯 issue：
- sebastianbergmann/phpunit#3526

.travis.yml

@@ -1,4 +1,5 @@
 language: php
+
 php:
   - '5.6'
   - '7.0'
@@ -8,11 +9,23 @@ php:
   - '7.4'
   - '8.0'
 
+jobs:
+  include:
+    - php: 5.6
+    - php: 7.0
+    - php: 7.1
+    - php: 7.2
+    - php: 7.3
+    - php: 7.4
+    - php: 8.0
+      env: COMPOSER_FLAGS="--ignore-platform-reqs"
+
+
 notifications:
   email: false
 
 install:
-  - composer install --prefer-dist --optimize-autoloader
+  - composer install --prefer-dist --optimize-autoloader $COMPOSER_FLAGS
   - composer show
 
 script: