I found for a similar problem (Rubymine 6.3.1 and Ruby 2.1.1) that I had to format .ruby-version in the following way:
ruby-2.1.1
Note the prefix 'ruby-" to the version number. I'm not sure whether this is an RVM, Ruby, Rubymine, or some other issue that caused this problem, but the fix I mentioned worked on my Mac.