Add FPDF and FPDI dependencies in composer.json

[Seb33300]

FPDF and FPDI should be added to composer.json requirements and removed from this repository. So we can get updates easily.

{
    "require": {
        "setasign/fpdi-fpdf": "1.6.1"
    }
}

See: https://github.com/Setasign/FPDI

[seyfer]

+100500. very important