Symfony 4 compat

[Jean85]

I've worked on a fork of my own to make this bundle compatible with Symfony 4. This is a minimum-effort work, since I did not delve in the advanced features like dynamic image embedding, since I do not use them.

This PR merges all my fork, which consists in the Symfony 4 milestone:

• facile-it/PdfBundle#1: require PHP 7.1; change the dir structure to use PSR-4 and src/tests dirs
• facile-it/PdfBundle#2: replace symfony/symfony requirement with explicit components requirements
• facile-it/PdfBundle#3: allow sensio/framework-extra-bundle 5
• facile-it/PdfBundle#4: update PHPUnit to 7.5/8
• facile-it/PdfBundle#5: bump all Symfony requirements from ^2.3|^3.0 to ^3.4|^4.0
• facile-it/PdfBundle#6: apply the Symfony code style

[sawmurai]

Any progress on this one? :)

[Jean85]

I'm using this fork in production. It works, but I'm not using it thoroughly with all functionalities, so I can't vouch for it.