support source_url, issues_url

In Chef 12, two new [cookbook metadata methods](https://github.com/opscode/chef/blob/9760d40e2f924e8fdd5b42760c4fb775572fb777/RELEASE_NOTES.md#new-cookbook-metadata-attributes-for-supermarket) for specifying the source and issues URLs are available.

These are not yet supported by Ridley:

```
An error occurred while reading the Berksfile:

  Could not parse `/Users/jtimberman/Development/cookbooks/mosh-cookbook/metadata.rb': undefined method `source_url' for #<Ridley::Chef::Cookbook::Metadata:0x007fbb6d0a5368>
```

Adding this guard around the methods works around the problem for now:

```ruby
unless defined?(Ridley::Chef::Cookbook::Metadata)
  source_url       'https://github.com/jtimberman/mosh-cookbook'
  issues_url       'https://github.com/jtimberman/mosh-cookbook/issues'
end
```