Fix appending .erb to template filename.

Blocking bug for Learn Chef tutorials.

Not sure why this issue wasn't turning up in spec tests. My best guess is that mixlib/cli's option parsing returns frozen strings and our stubs in `spec/unit/command/generator_commands_spec.rb` don't touch the option parsing code. Perhaps we should add functional/integration tests to chef-dk.

Checked other file generators and none try to directly modify `new_file_basename`.

\cc @opscode/client-eng