Update PHPUnit to ^6.0.

Author: bycosta

composer.json

@@ -18,7 +18,7 @@
         "php": ">=5.4",
         "behat/behat": "~3.0",
         "guzzlehttp/guzzle": "~6",
-        "phpunit/phpunit": ">=4.0"
+        "phpunit/phpunit": "^6.0"
     },
     "require-dev": {
         "symfony/process": "~2.1",

src/Context/WebApiContext.php

@@ -14,7 +14,7 @@
 use Behat\Gherkin\Node\TableNode;
 use GuzzleHttp\ClientInterface;
 use GuzzleHttp\Exception\RequestException;
-use PHPUnit_Framework_Assert as Assertions;
+use PHPUnit\Framework\Assert as Assertions;
 use GuzzleHttp\Psr7\Request;
 use Psr\Http\Message\ResponseInterface;