Slim :shortcut string values are deprecated

We have this error message almost everywhere:

    Slim :shortcut string values are deprecated, use hash like { '#' => { :tag => 'div', :attr => 'id' } }

WARNING! 
This pull request crashes tests because of Skim gem (which is a development dependency): it requires Slim 1.2.2, but there was no such feature in such old version. Here is an issue about this https://github.com/jfirebaugh/skim/issues/23, so skim is not going to support newer versions of Slim.