Support for mPDF 7.x

[KacerCZ]

Hi,
fix of issue #37 used class \Mpdf\Mpdf.
Later pull request #76 changed used class to \Mpdf.

mPDF up to version 6.1 uses class \mPDF.
From version 7.0 was added namespace and main class was renamed to \Mpdf\Mpdf, see https://github.com/mpdf/mpdf/blob/development/CHANGELOG.md

Is would be great if both versions could be supported, because mPDF 6.1 will not get support for PHP 7.1.

[PowerKiKi]

Thank you for the clarification. Clearly going back to non-namespaced code was a mistake. It should not have been merged that way.

Support for mPDF 6 must be dropped in favor of mPDF 7. Because mPDF 7 support all the same PHP versions as us. And we really don't have the resource to support two different versions.

Would you be able to submit a PR for that ?

[KacerCZ]

PR with fix is here - #149