Reverted increasing PHPUnit constraint

Author: DavertMik

composer.json

@@ -12,7 +12,7 @@
     "require": {
         "php": ">=7.0.0",
         "myclabs/deep-copy": "~1.1",
-        "phpunit/phpunit": "^6.0|^7.0"
+        "phpunit/phpunit": "^6.0"
     },
     "autoload": {
         "psr-0": {