[BUGFIX] Fix Composer-related build failure with PHP 7.0 (#52)

Composer tries to install doctrine/instantiator 1.1.0 on PHP 7.0,
but this version requires PHP ^7.1. This seems to be a bug in Composer,
and requiring the latest PHP-7.0-compatible version of
phpunit/phpunit-mock-objects works around this issue.

Author: oliverklee

composer.json

@@ -31,7 +31,8 @@
         "roave/security-advisories": "dev-master"
     },
     "require-dev": {
-        "phpunit/phpunit": "^6.5.0",
+        "phpunit/phpunit": "^6.5.6",
+        "phpunit/phpunit-mock-objects": "^5.0.6",
         "guzzlehttp/guzzle": "^6.3.0",
         "squizlabs/php_codesniffer": "^3.2.0",
         "phpstan/phpstan": "^0.7.0",