Fix error message on plugin reload

When reloading Vdebug (ie as a result of a :BundleUpdate in Vundle) the following error message was being displayed:

    Error detected while processing /home/$USER/.vim/bundle/vdebug/plugin/vdebug.vim:
    line  165:
    E122: Function vdebug:edit already exists, add ! to replace it 

This fixes that.