Update phpunit to stable

We can not switch to 6.x to have API compatible with 4.8 for older PHP versions.

Signed-off-by: Michal Čihař <michal@cihar.com>

Author: nijel

composer.json

@@ -20,8 +20,8 @@
         "ext-mbstring": "*"
     },
     "require-dev": {
-        "phpunit/php-code-coverage": "~2.0 || ~3.0",
-        "phpunit/phpunit": "~4.8 || ~5.1"
+        "phpunit/php-code-coverage": "*",
+        "phpunit/phpunit": "~4.8 || ~5.7"
     },
     "conflict": {
         "phpmyadmin/motranslator": "<3.0"