RubyMine 7.1:

Settings -> Editor -> General -> Smart Keys -> Unindent -> To nearest indent position

Rails Long Term Support

Rails LTS provides security patches for old versions of Ruby on Rails (2.3, 3.2, 4.2 and 5.2)

RubyMine 7.0:

Settings -> Editor -> General -> Smart Keys -> Backspace smart indent -> uncheck

Rubymine 7: Howto disable backspace deletes lines instead of whitespaces