Update code standards to PHP 7.0+

[TavoNiievez]

composer.json

• Added "ext-json": "*" because json_encode is used in seeNumRecords.

src/Codeception/Lib/Connector/Symfony.php

• Added strict types: declare(strict_types=1);
• array_keys was used in rebootKernel's foreach because it was only necessary to know the keys in that loop.

src/Codeception/Module/Symfony.php

• Added strict types: declare(strict_types=1);
• array_keys was used in _after's foreach because it was only necessary to know the keys in that loop.
• Imported classes with the use statement whenever possible
• self::class instead of __CLASS__.
• Added types to the arguments whenever possible, and explicit type cast when it was necessary.
• Used PHP if/else shorthand in seeAuthentication and dontSeeAuthentication.