Install wkhtmltox via rpm

Update wkhtmltopdf recipe for the 12.1 release.  (distributed as .rpm)

Add amd64 .rpms for centos5 and 6.

Debian and Ubuntu are not supported because `package` doesn't do dependency resolution on those platforms.  (That's what I was worried about when I pushed the first version.)  Using the `gdebi` cookbook might work, but I wasn't sure going down that route would be worth the effort.

Also note likely Amazon linux issue noted here:
https://github.com/sportngin-cookbooks/wkhtmltopdf/blob/wkhtmltox/attributes/default.rb#L12
amazon linux is RHEL

https://github.com/opscode/ohai/blob/master/lib/ohai/plugins/linux/platform.rb#L116