see if looser phpunit v6 works on travis

Author: ashnazg

.travis.yml

@@ -10,8 +10,7 @@ matrix:
     - php: nightly
 
 install:
-  - travis_retry composer install --no-interaction --prefer-dist --optimize-autoloader --no-dev # phpunit won't install at same time due to ReflectionCommon inside it
-  - travis_retry composer require --dev phpunit/phpunit # so install phpunit separately
+  - travis_retry composer install --no-interaction --prefer-dist --optimize-autoloader
   - travis_retry wget https://phar.io/releases/phive.phar
 
 script:

composer.json

@@ -24,6 +24,6 @@
         }
     },
     "require-dev": {
-        "phpunit/phpunit": "^6.5"
+        "phpunit/phpunit": "^6"
     }
 }