Fixed some style and some minor issues

* Added file header in all bundle php files
* Added most of phpdoc in all methods, even in tests
* Grouped all Unit FOSHttpCacheExtensionTest cases in just one case, using dataProvider
  instead of repeating same logic in all methods
* Fixed indents and some style using php-cs-fixer
* Completed .travis.yml with php5.6 and hhvm, allowing hhvm failure
* Added phpunit dependency in composer require-dev zone
* Renamed phpunit.xml.dist to phpunit.xml
the recommended installation way for PHPUnit is the phar AFAIK