dw2pdf based on mPDF 6.0

Last version based at the 6.0 release of mPDF. (this version supports PHP < 7.0)

dw2pdf based at mPDF 5.7.4

Version based at the 5.7 branch of mPDF.
It serves as fallback for if the mPDF 6.0 has still some not foreseen issues.