Ensure variable in mail is evalled

I received this snippet in the mail:

```
Content-Type: text/plain;
 charset=UTF-8
Content-Transfer-Encoding: 7bit

Project #{@old_path_with_namespace} was moved to another location

The project is now located under
```

This MR fixes it.

Notes:

* I did a quick grep on other 'text.erb' files, but this was the only one with this type of error
* I made no changes to the specs. The only [spec](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/spec/mailers/notify_spec.rb#L412) I could find passes because (I assume) the correct value is in the html-part

See merge request !1706