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
Instead of adding conditionals to the attributes file, why don't we just check for the files existence here instead of basing it on a built up string:

```ruby
# attributes/default.rb

wk = node['wkhtmltox']
default['wkhtmltox']['package_file'] = "wkhtmltox-#{wk['version']}_linux-#{wk['release']}-#{wk['arch']}.rpm"

# recipes/default.rb

raise "No package_file '#{package_file}' was found" unless File.exists?(package_file)
```

You might have to do some directory traversal that I am not going to test here, but you should be able to determine if the package exists in the repo.