use updated URL for wkhtmltopdf downloads

[ashkulz]

Hello,

wkhtmltopdf has moved the downloads provider from SourceForge to gna.org. As mentioned in the linked message, all earlier downloads have been removed from SourceForge and you are strongly advised to migrate to the new URLs. The following files in your repository will need to be updated:

• Dockerfile

It is recommended to either subscribe to the mailing list or Twitter feed to receive updates on new releases and further changes.

Thanks,
wkhtmltopdf maintainer

[sharoonthomas]

@ashkulz thanks for the headsup.