Add upstream wkhtmltopdf package

[yajo]

The package from Debian and Ubuntu repositories does not work fine with Odoo, and thus cannot generate PDF reports.

The upstream one (from http://wkhtmltopdf.org/downloads.html) is needed.

[florentx]

It seems that the package in Debian testing is OK: 0.12.1
Only the package in Ubuntu 14.04 is outdated: 0.9.9

[florentx]

I guess that this workaround should do the trick on Ubuntu:
https://www.odoo.com/fr_FR/forum/help-1/question/wkhtmltopdf-on-v8-not-found-48208#answer-55374

I will update the Dockerfile accordingly.