Upgrade PHPUnit to latest 4.6

Author: OndraM

composer.json

@@ -1,6 +1,6 @@
 {
   "name": "facebook/webdriver",
-  "description": "A php client for WebDriver",
+  "description": "A PHP client for WebDriver",
   "keywords": ["webdriver", "selenium", "php", "facebook"],
   "homepage": "https://github.com/facebook/php-webdriver",
   "type": "library",
@@ -14,7 +14,7 @@
     "php": ">=5.3.19"
   },
   "require-dev": {
-    "phpunit/phpunit": "3.7.*"
+    "phpunit/phpunit": "4.6.*"
   },
   "suggest": {
     "phpdocumentor/phpdocumentor": "2.*"