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",