[php-symfony] Set required PHP version ^7.1.3 (#6181)

* Set PHP 7.1.3 required version

I've tried to specify ^7.0 version at first, but main package which is
symfony/framework-bundle@v4.4.8 requires PHP ^7.1.3.

* Bump Symfony FrameworkBundle to ^4.4.8

Current Symfony Framework stable version is v5.0.8, but I guess it
requires significant codebase upgrade, so I've sticked with 4.4.8 which
shouldn't cause any breaking changes. Old requirement was ^3.3|^4.1
which compatible with 4.4.8.

* Bump PHPUnit version to ^7.0

PHPUnit 8.x version required PHP ^7.2, so I'm setting 7.x version to
support PHP 7.1.
There is new way to specify Kernel class, related PR:
https://github.com/symfony/symfony/pull/22668

* Bump PHP CS Fixer version to ^2.16.3

Configuration and all renamed rules fixed.
Config file renamed to .php_cs.dist as recommended in migration guide.
Migration guide from 1.x to 2.x:
https://github.com/FriendsOfPHP/PHP-CS-Fixer/blob/master/UPGRADE.md#config-file

* Remove PHP_CodeSniffer package

Second linter doesn't make sense. I think Symfony user would prefer
PHP CS Fixer over PHP_CodeSniffer because first one maintained by Symfony
members.

* Remove satooshi/php-coveralls package from Composer

This package is abandoned and Coveralls recommends to install it directly
in Travis-CI task script.

* Update Travic-CI config

I've changed test versions to PHP 7.1.3 and 7.2. PHPUnit generates
coverage report in report/logs/clover.xml file. Then PHP CS Fixer runs
with --dry-run option to not override anything just to show coding style
errors.

* Add basic Coveralls config

This is basic recommended config for a PHP based project.

* Add symfony/yaml package

This package was part of satooshi/php-coveralls, now it should be
defined as dev dependency.

* Do not commit composer.lock

I think committed composer.lock can cause CI errors while tests on fresh
installs are better.

* Remove confusing Ruby comment

samples/server/petstore/php-symfony/SymfonyBundle-php/DependencyInjection/Compiler/OpenAPIServerApiPass.php

@@ -2,7 +2,7 @@
 /**
  * OpenAPIServerApiPass
  *
- * PHP version 5
+ * PHP version 7.1.3
  *
  * @category Class
  * @package  OpenAPI\Server\DependencyInjection\Compiler

samples/server/petstore/php-symfony/SymfonyBundle-php/DependencyInjection/OpenAPIServerExtension.php

@@ -2,7 +2,7 @@
 /**
  * OpenAPIServerExtension
  *
- * PHP version 5
+ * PHP version 7.1.3
  *
  * @category Class
  * @package  OpenAPI\Server\DependencyInjection