Fix eruby indentation when 'shiftwidth' is 0

For https://github.com/vim-ruby/vim-ruby/issues/239

This change is based on commit
9d95f296a7afde76361130217304d75004c0d8f9
Merge pull request #240 from roryokane/eruby-indent-sw-0

Fix eruby indentation when 'shiftwidth' is 0