Uninstall the wkhtmltopdf package:
apt-get remove wkhtmltopdf --purge
cd /usr/local/bin
sudo curl -C - -O http://download.gna.org/wkhtmltopdf/obsolete/linux/wkhtmltopdf-0.9.9-static-amd64.tar.bz2
sudo tar -xvjf wkhtmltopdf-0.9.9-static-amd64.tar.bz2
rm wkhtmltopdf-0.9.9-static-amd64.tar.bz2
ln -s wkhtmltopdf-amd64 wkhtmltopdf
In your initializer
WickedPdf.config = { :exe_path => "/usr/local/bin/wkhtmltopdf" }
http://wkhtmltopdf.org/obsolete-downloads.html