Update to rubocop 0.27

I disabled MultilineOperationIndentation because it didn't make
sense. It wanted to reformat this:

    foo = bar
      .baz

Into this:

    foo = bar
          .baz
Fix #228; switch to kramdown

This should populate our documentation on RubyDoc.info.

See also #27 and #9.
Using model:model
Removed unused imports
Merge branch 'model_model' of github.com:Willianvdv/active_interaction into model_model