Fix eruby indentation when 'shiftwidth' is 0

For [#239, Indentation fails when 'shiftwidth' is 0](https://github.com/vim-ruby/vim-ruby/issues/239)

This change is based on the commit [Set correct "shiftwidth" value](https://github.com/vim-ruby/vim-ruby/commit/9d95f296a7afde76361130217304d75004c0d8f9). That change only fixes Ruby files, not `.erb` files. I tested indenting an erb file, and saw that indentation didn’t work before this change and does work after this change.