Previous version of Formtastic was giving following deprecation warning:
`#column_for_attribute` will return a null object for non-existent
columns in Rails 5. Use #has_attribute? if you need to check for
an attribute's existence.
Guide followed for upgrade: https://github.com/justinfrench/formtastic/wiki/Upgrading-to-Formtastic-3.1
formtastic-bootstrap is upgraded for compatibility with current Formtastic.
