Properly expand cookbook_path

Having something like `cookbook_path [ './cookbooks' ]` in *~.chef/knife.rb* results in this error during `knife cookbook -V -V github install [...]`:

    STDERR: mv: cannot move '_tmp_chef_nginx' to 'cookbooks/nginx': No such file or directory

[line 127 of cookbook_github_install.rb](https://github.com/nphase/knife-github-cookbooks/blob/master/lib/chef/knife/cookbook_github_install.rb#L127) triggers this error, as it runs `mv` from a temporary directory.