undefined method `use_transactional_tests=' for #<RSpec::Core::Configuration:0x007fccf515e338> (NoMethodError)

The deprecation warning and suggestion you are getting is from Rails, not RSpec or the rspec-rails gem. The rspec-rails gem implements use_transactional_fixtures= and turns around and calls the same method in Rails. It does not implement use_transactional_tests=, so you're getting that error message when you made that change.

Unless the rspec-rails gem is upgraded to support the new Rails 5 features, you'll need to set the Rails configuration directly - and as of rspec-rails 3.8 it does not.

You need to keep using config.use_transactional_fixtures = true in rspec configure for rails 5 beta (at least till the official release, not sure what are the rspec team plans on supporting this). The fix is already on master branch, here is the line fixing it.

Set the rspec gems on master branch:

%w[rspec-core rspec-expectations rspec-mocks rspec-rails rspec-support].each do |lib|
    gem lib, git: "https://github.com/rspec/#{lib}.git", branch: 'master'
end