Fix Dashboard notices

Hey again @paulgibbs,

Thanks for the speedy merge of my other PR :smile:! I was working on frontend stuff for most of today and only just realised there are a few notices in the admin as well.

```
Strict Standards:  call_user_func_array() expects parameter 1 to be a valid callback, non-static method DP_Welcome_Pack_Admin::setup_menu() should not be called statically in /vagrant/wp/wp-includes/plugin.php on line 470

Strict Standards:  call_user_func_array() expects parameter 1 to be a valid callback, non-static method DP_Welcome_Pack_Admin::email_maybe_save() should not be called statically in /vagrant/wp/wp-includes/plugin.php on line 470

Strict Standards:  call_user_func_array() expects parameter 1 to be a valid callback, non-static method DP_Welcome_Pack_Admin::email_subject_placeholder() should not be called statically in /vagrant/wp/wp-includes/plugin.php on line 192
```

I've attached a PR to resolve those as well and have suggested a minor version bump but I'm happy to revert that if you don't want to bump it!

Thanks again!