Lock down PHPUnit for PHP 5.5
Lock down PHPUnit for PHP 5.5

Since Composer defined lowest version is now PHP 5.5.9.
PHPUnit 5.0 require PHP 5.6.

wallabag/wallabag,shtrom/wallabag,X-dark/wallabag,shtrom/wallabag,pmichelazzo/wallabag,X-dark/wallabag,X-dark/wallabag,egilli/wallabag,shtrom/wallabag,pmichelazzo/wallabag,egilli/wallabag,egilli/wallabag,pmichelazzo/wallabag,X-dark/wallabag,pmichelazzo/wallabag,wallabag/wallabag,shtrom/wallabag,shtrom/wallabag,X-dark/wallabag,egilli/wallabag,pmichelazzo/wallabag,egilli/wallabag,wallabag/wallabag