[ticket/14947] Updates phpunit to 4.8

$ composer update phpunit/phpunit --with-dependencies
Dependency "symfony/yaml" is also a root requirement, but is not explicitly whitelisted. Ignoring.
Loading composer repositories with package information
Updating dependencies (including require-dev)
  - Installing sebastian/global-state (1.1.1)
    Loading from cache

  - Installing doctrine/instantiator (1.0.5)
    Loading from cache

  - Removing phpunit/phpunit-mock-objects (2.1.5)
  - Installing phpunit/phpunit-mock-objects (2.3.8)
    Loading from cache

  - Removing phpunit/php-file-iterator (1.3.4)
  - Installing phpunit/php-file-iterator (1.4.2)
    Loading from cache

  - Installing phpdocumentor/reflection-docblock (2.0.4)
    Downloading: 100%

  - Installing phpspec/prophecy (v1.6.2)
    Loading from cache

  - Removing phpunit/phpunit (4.1.6)
  - Installing phpunit/phpunit (4.8.31)
    Downloading: 100%

sebastian/global-state suggests installing ext-uopz (*)
phpdocumentor/reflection-docblock suggests installing dflydev/markdown (~1.0)
phpdocumentor/reflection-docblock suggests installing erusev/parsedown (~1.0)
Writing lock file
Generating autoload files
> echo 'You MUST manually modify the clean-vendor-dir target in build/build.xml when adding or upgrading dependencies.'
You MUST manually modify the clean-vendor-dir target in build/build.xml when adding or upgrading dependencies.

PHPBB3-14947

phpBB/composer.json

@@ -55,7 +55,7 @@
 		"laravel/homestead": "~2.2",
 		"phing/phing": "2.4.*",
 		"phpunit/dbunit": "1.3.*",
-		"phpunit/phpunit": "4.1.*",
+		"phpunit/phpunit": "^4.1",
 		"sami/sami": "1.*",
 		"squizlabs/php_codesniffer": "2.*",
 		"symfony/browser-kit": "^2.8",