Allow specification of a package version

On ubuntu 12.04 I've gotten into a situation where the preferred version of
nginx installed by apt-get is not the version I want.  For instance looking at
apt-cache you can see the 1.2.9 version is the one that gets picked.

  $ sudo apt-cache madison nginx
     nginx | 1:1.2.9-1bbox1+ng~precise1 | http://ppa.launchpad.net/brightbox/ruby-ng/ubuntu/ precise/main amd64 Packages
     nginx | 1.6.0-1~precise | http://nginx.org/packages/ubuntu/ precise/nginx amd64 Packages
     nginx | 1.1.19-1ubuntu0.6 | http://us-west-2.ec2.archive.ubuntu.com/ubuntu/ precise-updates/universe amd64 Packages
     nginx | 1.1.19-1ubuntu0.5 | http://security.ubuntu.com/ubuntu/ precise-security/universe amd64 Packages
     nginx |   1.1.19-1 | http://us-west-2.ec2.archive.ubuntu.com/ubuntu/ precise/universe amd64 Packages
     nginx |   1.1.19-1 | http://us-west-2.ec2.archive.ubuntu.com/ubuntu/ precise/universe Sources
     nginx | 1.1.19-1ubuntu0.6 | http://us-west-2.ec2.archive.ubuntu.com/ubuntu/ precise-updates/universe Sources
     nginx | 1.1.19-1ubuntu0.5 | http://security.ubuntu.com/ubuntu/ precise-security/universe Sources
     nginx | 1.6.0-1~precise | http://nginx.org/packages/ubuntu/ precise/nginx Sources

I can install the 1.6.0-1 version using apt-cache like this:

  $ sudo apt-get install nginx=1.6.0-1~precise

However the current cookbook options do not appear to support this type of
installation.  When I set options like this:

  node['nginx']['package_name'] = "nginx=1.6.0-1~precise"

I get an error like:

  No version specified, and no candidate version available for nginx=1.6.0-1~precise

Using the version attribute on the package resource seems to make this work.