Update minimum PHPUnit version to make sure Prophecy is available

Author: sagikazarmark

composer.json

@@ -12,7 +12,7 @@
     ],
     "require": {
         "php": ">=5.4",
-        "phpunit/phpunit": "^4.4",
+        "phpunit/phpunit": "^4.5",
         "php-http/discovery": "^0.4",
         "php-http/utils": "^0.2",
         "php-http/httplug": "~1.0@dev",