Fix rdoc formatter

Running `GLI_DEBUG=true bin/my-cli _doc` fails with the following stack trace. This patch fixes it.

I didn't check why the tests aren't failing even though [todo.feature](https://github.com/davetron5000/gli/blob/gli-2/features/todo.feature#L546) has a test for this formatter.

```
error: undefined method `exe_name' for nil:NilClass
/opt/boxen/rbenv/versions/1.9.3-p547/gemsets/my-cli/gems/gli-2.12.2/lib/gli/commands/rdoc_document_listener.rb:23:in `program_desc': undefined method `exe_name' for nil:NilClass (NoMethodError)
        from /opt/boxen/rbenv/versions/1.9.3-p547/gemsets/my-cli/gems/gli-2.12.2/lib/gli/commands/doc.rb:37:in `document'
        from /opt/boxen/rbenv/versions/1.9.3-p547/gemsets/my-cli/gems/gli-2.12.2/lib/gli/commands/doc.rb:26:in `block in initialize'
        from /opt/boxen/rbenv/versions/1.9.3-p547/gemsets/my-cli/gems/gli-2.12.2/lib/gli/command_support.rb:126:in `call'
        from /opt/boxen/rbenv/versions/1.9.3-p547/gemsets/my-cli/gems/gli-2.12.2/lib/gli/command_support.rb:126:in `execute'
        from /opt/boxen/rbenv/versions/1.9.3-p547/gemsets/my-cli/gems/gli-2.12.2/lib/gli/app_support.rb:290:in `block in call_command'
        from /opt/boxen/rbenv/versions/1.9.3-p547/gemsets/my-cli/gems/gli-2.12.2/lib/gli/app_support.rb:303:in `call'
        from /opt/boxen/rbenv/versions/1.9.3-p547/gemsets/my-cli/gems/gli-2.12.2/lib/gli/app_support.rb:303:in `call_command'
        from /opt/boxen/rbenv/versions/1.9.3-p547/gemsets/my-cli/gems/gli-2.12.2/lib/gli/app_support.rb:81:in `run'
```