[BUG][php-symfony] Composer installation fails under PHP 7.2

Created by: ybelenko

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

During review of #6078 found following installation problem:

$ composer install
Deprecation warning: Your package name GIT_USER_ID/GIT_REPO_ID is invalid, it should not contain uppercase characters. We suggest using git_user_id/git_repo_id instead. Make sure you fix this as Composer 2.0 will error.
Loading composer repositories with package information
Updating dependencies (including require-dev)
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - friendsofphp/php-cs-fixer v1.13.3 requires php ^5.3.6 || >=7.0 <7.2 -> your PHP version (7.2.21) does not satisfy that requirement.
    - friendsofphp/php-cs-fixer v1.13.2 requires php ^5.3.6 || >=7.0 <7.2 -> your PHP version (7.2.21) does not satisfy that requirement.
    - friendsofphp/php-cs-fixer v1.13.1 requires php ^5.3.6 || >=7.0 <7.2 -> your PHP version (7.2.21) does not satisfy that requirement.
    - friendsofphp/php-cs-fixer v1.13.0 requires php ^5.3.6 || >=7.0 <7.2 -> your PHP version (7.2.21) does not satisfy that requirement.
    - friendsofphp/php-cs-fixer v1.12.4 requires php ^5.3.6 || >=7.0 <7.2 -> your PHP version (7.2.21) does not satisfy that requirement.
    - friendsofphp/php-cs-fixer v1.12.3 requires php ^5.3.6 || >=7.0 <7.2 -> your PHP version (7.2.21) does not satisfy that requirement.
    - friendsofphp/php-cs-fixer v1.12.2 requires php ^5.3.6 || >=7.0 <7.2 -> your PHP version (7.2.21) does not satisfy that requirement.
    - friendsofphp/php-cs-fixer v1.12.1 requires php ^5.3.6 || >=7.0 <7.2 -> your PHP version (7.2.21) does not satisfy that requirement.
    - friendsofphp/php-cs-fixer v1.12.0 requires php ^5.3.6 || >=7.0 <7.2 -> your PHP version (7.2.21) does not satisfy that requirement.
    - Installation request for friendsofphp/php-cs-fixer ~1.12 -> satisfiable by friendsofphp/php-cs-fixer[v1.12.0, v1.12.1, v1.12.2, v1.12.3, v1.12.4, v1.13.0, v1.13.1, v1.13.2, v1.13.3].

openapi-generator version

Current master branch. Which is more important is tested PHP version:

$ php -v
PHP 7.2.21 (cli) (built: Aug 11 2019 20:58:12) ( NTS )
Copyright (c) 1997-2018 The PHP Group
Zend Engine v3.2.0, Copyright (c) 1998-2018 Zend Technologies
    with Zend OPcache v7.2.21, Copyright (c) 1999-2018, by Zend Technologies
    with Xdebug v2.7.2, Copyright (c) 2002-2019, by Derick Rethans

OpenAPI declaration file content or url

Build under samples directory.

Command line used for generation

Build under samples directory.

Steps to reproduce

$ cd samples/server/petstore/php-symfony/SymfonyBundle-php
$ composer install

Suggest a fix

All dependencies should be specified as * or build should be tested under different PHP versions.

cc @jebentier @dkarlovi @mandrean @jfastnacht @ackintosh @renepardon cc PR's author @reznikartem