Exclude PHPUnit-6.5

PHPUnit-6.5 did change the signature of TestCase::getMockBuilder() by
returning an object from phpunit-mock-object-5 instead of
phpunit-mock-object-4. This broke this library:

  PHP Fatal error: Interface 'PHPUnit_Framework_MockObject_Matcher_Invocation'
  not found in /.../vendor/php-mock/php-mock-phpunit/classes/DefaultArgumentRemover.php
  on line 15

This commit adds a <phpunit-6.5 constraint to fix the current release.
PHPUnit-6.5 is currently not supported by this library. I wait for
sebastianbergmann/phpunit#2893 to decide how to support PHPUnit-6.5.

See also: #21
See also: sebastianbergmann/phpunit#2893

Author: malkusch

composer.json

@@ -18,7 +18,7 @@
     },
     "require": {
         "php": ">=7",
-        "phpunit/phpunit": "^6",
+        "phpunit/phpunit": "^6 <6.5",
         "php-mock/php-mock-integration": "^2"
     },
     "archive": {