Resolve nil error for berksfile in extract_local

Line 35 originally called `berksfile.cookbook_list` even if `berksfile`
was never instantiated. This commit checks if the berksfile object has
been created before calling `ExtractLocal.resolve_cookbooks` with the
appropriate parameters.
Merge pull request #37 from crymer11/patch-2

Resolve nil error for berksfile in extract_local