Remove phpunit 6.0 from dev-dependency

phpunit 6.0 removed from dev-dependency because it requires php >=7.0

.travis.yml

@@ -8,4 +8,4 @@ before_script:
   - travis_retry composer install --prefer-source --no-interaction
 
 script:
-  - vendor/bin/phpunit
+  - phpunit --bootstrap=vendor/autoload.php tests

composer.json

@@ -25,7 +25,6 @@
         "laravel/browser-kit-testing": "^2.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "~6.0",
         "orchestra/testbench": "~3.0"
     },
     "extra": {