Update for internal packages

Add a step in the Nuget dependency resolver to update the internal packages to a new version.

The new version is taken from the `bozorc.rb` file for each project. This has been tested with Underwriting, where the `bozorc.rb` looks like this:

    resolve_dependencies_with :nuget do |n|
      n.source '\\\\LDN-SERVER-A.ldn.zopa.com\\tech\\Zopa Packages'
      n.packages_to_update ({ "Zopa.Common" => "4.22.1", "Zopa.Common.Testing" => "4.22.1", "Zopa.Equifax" => "4.22.1" })
    end

Third line above is the new stuff.

Once this is merged, then you can just update the `bozorc.rb` file in each project and run `bin/bozo prepare`. That would take care of updating all references to each package.